Support markdown link navigation when duplicate slugs exist

Fixes #59711

For a md document:

```md
# a

# a

- [a](#a)
- [next a](#a-1)
```

You can now click on the second link in the editor to navigate to the second `a` header. It is identified by being suffixed with `-1`.

extensions/markdown-language-features/src/tableOfContentsProvider.ts

@@ -51,11 +51,23 @@ export class TableOfContentsProvider {
 		const toc: TocEntry[] = [];
 		const tokens = await this.engine.parse(document.uri, document.getText());
 
+		const slugCount = new Map<string, number>();
+
 		for (const heading of tokens.filter(token => token.type === 'heading_open')) {
 			const lineNumber = heading.map[0];
 			const line = document.lineAt(lineNumber);
+
+			let slug = githubSlugifier.fromHeading(line.text);
+			if (slugCount.has(slug.value)) {
+				const count = slugCount.get(slug.value)!;
+				slugCount.set(slug.value, count + 1);
+				slug = githubSlugifier.fromHeading(slug.value + '-' + (count + 1));
+			} else {
+				slugCount.set(slug.value, 0);
+			}
+
 			toc.push({
-				slug: githubSlugifier.fromHeading(line.text),
+				slug,
 				text: TableOfContentsProvider.getHeaderText(line.text),
 				level: TableOfContentsProvider.getHeaderLevel(heading.markup),
 				line: lineNumber,

extensions/markdown-language-features/src/test/tableOfContentsProvider.test.ts

@@ -106,4 +106,25 @@ suite('markdown.TableOfContentsProvider', () => {
 		assert.strictEqual((await provider.lookup('Заголовок-header-3'))!.line, 4);
 		assert.strictEqual((await provider.lookup('Заголовок'))!.line, 5);
 	});
+
+	test('Lookup should support suffixes for repeated headers', async () => {
+		const doc = new InMemoryDocument(testFileName, `# a\n# a\n## a`);
+		const provider = new TableOfContentsProvider(createNewMarkdownEngine(), doc);
+
+		{
+			const entry = await provider.lookup('a');
+			assert.ok(entry);
+			assert.strictEqual(entry!.line, 0);
+		}
+		{
+			const entry = await provider.lookup('a-1');
+			assert.ok(entry);
+			assert.strictEqual(entry!.line, 1);
+		}
+		{
+			const entry = await provider.lookup('a-2');
+			assert.ok(entry);
+			assert.strictEqual(entry!.line, 2);
+		}
+	});
 });