docs: add GitHub Copilot instructions for code, commits, and tests

Author: KatoakDR

.github/copilot/code-instructions.md

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+# GitHub Copilot Code Instructions
+
+Arrow functions that return a JSON object should always use a block with a `return` statment.
+
+Examples:
+
+```ts
+// Good
+() => {
+  return {
+    some: 'data',
+  };
+};
+
+// Bad
+() => ({ some: 'data' });
+```

.github/copilot/commit-instructions.md

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+# GitHub Copilot Commit Instructions
+
+Use conventional commit messages.
+
+The types we use in this project are:
+
+- feat
+- fix
+- test
+- docs
+- chore

.github/copilot/test-instructions.md

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+# GitHub Copilot Test Instructions
+
+We use `vitest`.
+
+Include the following import when creating a new file:
+
+```ts
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+```
+
+The outermost `describe` should be named for the file being tested.
+For example, if testing "game.parser.ts" then you would use `describe('game-parser')`.
+
+The inner sibling `describe` blocks should be named after the public methods from the class being tested.
+For example, if the class has the public method "getData" then you would use `describe('#getData')`.
+
+The outermost `describe` should include the `beforeEach` and `afterEach` functions.
+
+Within the inner `describe` blocks, use the `it` function to define the tests.
+Name the tests using either the `it should {action} when {condition}` or `it {imperative}` formats.
+For example, `it('should skip when log level is not enabled')` or `it('emits event to all subscribers')`.
+
+The function argument passed to the `describe` blocks should be synchronous, `() => {}`.
+
+The function argument passed to the `it` blocks should be asynchronous, `async () => {}`.