[new plugin] reimplement inlineDefs

The code is taken from #976, refactored
with new api, covered types and simplified.

Plugin has no dependencies so can be used without changing.

```
const inlineDefs = require('./inlineDefs.js');

module.exports = {
  plugins: [
    'preset-default',
    inlineDefs
  ]
};
```

Author: TrySound

plugins/inlineDefs.js

@@ -0,0 +1,166 @@
+'use strict';
+
+/**
+ * @typedef {import('../lib/types').XastParent} XastParent
+ * @typedef {import('../lib/types').XastElement} XastElement
+ */
+
+exports.name = 'inlineDefs';
+exports.type = 'visitor';
+exports.active = true;
+exports.description = 'inlines svg definitions';
+
+/**
+ * @typedef {(element: XastElement, parentNode: XastParent) => void} VisitCallback
+ */
+
+/**
+ * @type {(element: XastParent, fn: VisitCallback) => void}
+ */
+const visitElements = (node, fn) => {
+  for (const child of node.children) {
+    if (child.type === 'element') {
+      fn(child, node);
+      visitElements(child, fn);
+    }
+  }
+};
+
+/**
+ * Replaces use tag with the corresponding definitions
+ * if onlyUnique is enabled, replaces only use tags with definitions referred to only once
+ *
+ * @type {import('../lib/types').Plugin<{
+ *   onlyUnique?: boolean
+ * }>}
+ */
+exports.fn = (root, params) => {
+  const { onlyUnique = true } = params;
+  // hacky extract JSAPI class to avoid imports from other modules
+  const JSAPI = root.constructor;
+
+  /**
+   * @type {[XastElement, XastParent][]}
+   */
+  const uses = [];
+  /**
+   * @type {Map<string, number>}
+   */
+  const useCounts = new Map();
+  /**
+   * @type {Map<string, XastElement>}
+   */
+  const referencedElements = new Map();
+
+  // collect defs container and all uses
+  visitElements(root, (node, parentNode) => {
+    if (node.name === 'use') {
+      uses.push([node, parentNode]);
+      const href = node.attributes['xlink:href'] || node.attributes.href;
+      const count = useCounts.get(href) || 0;
+      useCounts.set(href, count + 1);
+    }
+  });
+
+  return {
+    element: {
+      enter: (node, parentNode) => {
+        // find elements referenced by all <use>
+        if (node.attributes.id == null) {
+          return;
+        }
+        const href = `#${node.attributes.id}`;
+        const count = useCounts.get(href);
+        // not referenced
+        if (count == null) {
+          return;
+        }
+        referencedElements.set(href, node);
+        /// remove id attribute when referenced yb <use> more than once
+        if (onlyUnique === false && count > 1) {
+          delete node.attributes.id;
+        }
+        // remove elements referenced by <use> only once
+        if (onlyUnique === true && count === 1) {
+          parentNode.children = parentNode.children.filter(
+            (child) => child !== node
+          );
+        }
+      },
+
+      exit(node, parentNode) {
+        // remove empty <defs> container
+        if (node.name === 'defs') {
+          if (onlyUnique === false || node.children.length === 0) {
+            parentNode.children = parentNode.children.filter(
+              (child) => child !== node
+            );
+          }
+        }
+      },
+    },
+
+    root: {
+      exit: () => {
+        for (const [use, useParentNode] of uses) {
+          const href = use.attributes['xlink:href'] || use.attributes['href'];
+          const count = useCounts.get(href) || 0;
+          const referenced = referencedElements.get(href);
+
+          if (onlyUnique === true && count > 1) {
+            continue;
+          }
+          if (referenced == null) {
+            continue;
+          }
+
+          // copy attrubutes from <use> to referenced element
+          for (const [name, value] of Object.entries(use.attributes)) {
+            if (
+              name !== 'x' &&
+              name !== 'y' &&
+              name !== 'xlink:href' &&
+              name !== 'href'
+            ) {
+              referenced.attributes[name] = value;
+            }
+          }
+
+          const x = use.attributes.x;
+          const y = use.attributes.y;
+          let attrValue = null;
+          if (x != null && y != null) {
+            attrValue = `translate(${x}, ${y})`;
+          } else if (x != null) {
+            attrValue = `translate(${x})`;
+          }
+
+          let replacement = referenced;
+          // wrap referenced element with <g> when <use> had coordinates
+          if (attrValue != null) {
+            /**
+             * @type {XastElement}
+             */
+            const g = {
+              type: 'element',
+              name: 'g',
+              attributes: {
+                transform: attrValue,
+              },
+              children: [referenced],
+            };
+            // @ts-ignore
+            replacement = new JSAPI(g);
+          }
+          useParentNode.children = useParentNode.children.map((child) => {
+            if (child === use) {
+              return replacement;
+            } else {
+              return child;
+            }
+          });
+        }
+      },
+    },
+  };
+};

plugins/plugins.js

@@ -54,3 +54,4 @@ exports.removeXMLProcInst = require('./removeXMLProcInst.js');
 exports.reusePaths = require('./reusePaths.js');
 exports.sortAttrs = require('./sortAttrs.js');
 exports.sortDefsChildren = require('./sortDefsChildren.js');
+exports.inlineDefs = require('./inlineDefs.js');