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/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);
+		}
+	});
 });