builtins/0: add builtins function

jackson-jq/src/main/java/net/thisptr/jackson/jq/Scope.java

@@ -95,6 +95,16 @@ public Function getFunction(final String name, final int nargs) {
 		return getFunctionRecursive(name);
 	}
 
+	public Map<String, Function> getLocalFunctions() {
+		if (functions == null)
+			return new HashMap<>();
+		return new HashMap<>(functions);
+	}
+
+	public Scope getParentScope() {
+		return parentScope;
+	}
+
 	private Function getFunctionRecursive(final String name) {
 		if (functions != null) {
 			final Function q = functions.get(name);

jackson-jq/src/main/java/net/thisptr/jackson/jq/internal/functions/BuiltinsFunction.java

@@ -0,0 +1,33 @@
+package net.thisptr.jackson.jq.internal.functions;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.google.auto.service.AutoService;
+
+import net.thisptr.jackson.jq.BuiltinFunction;
+import net.thisptr.jackson.jq.Expression;
+import net.thisptr.jackson.jq.Function;
+import net.thisptr.jackson.jq.PathOutput;
+import net.thisptr.jackson.jq.Scope;
+import net.thisptr.jackson.jq.Version;
+import net.thisptr.jackson.jq.exception.JsonQueryException;
+import net.thisptr.jackson.jq.path.Path;
+
+@AutoService(Function.class)
+@BuiltinFunction("builtins/0")
+public class BuiltinsFunction implements Function {
+
+	@Override
+	public void apply(Scope scope, final List<Expression> args, final JsonNode in, final Path path, final PathOutput output, final Version version) throws JsonQueryException {
+		// root scope
+		while (scope.getParentScope() != null)
+			scope = scope.getParentScope();
+
+		final List<String> builtins = new ArrayList<>(scope.getLocalFunctions().keySet());
+		Collections.sort(builtins);
+		output.emit(scope.getObjectMapper().valueToTree(builtins), null);
+	}
+}