Reduce number of times MD docs are re-tokenized (#152674)

This change reduces the number of times we retokenize a markdown file by doing the following:

- Use `MdTableOfContentsProvider` in more places
- Introduce a `IMarkdownParser` interface that lets us drop in a caching version of the tokenizer

extensions/markdown-language-features/src/commands/reloadPlugins.ts

@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import { Command } from '../commandManager';
-import { MarkdownEngine } from '../markdownEngine';
+import { MarkdownItEngine } from '../markdownEngine';
 import { MarkdownPreviewManager } from '../preview/previewManager';
 
 export class ReloadPlugins implements Command {
@@ -12,7 +12,7 @@ export class ReloadPlugins implements Command {
 
 	public constructor(
 		private readonly webviewManager: MarkdownPreviewManager,
-		private readonly engine: MarkdownEngine,
+		private readonly engine: MarkdownItEngine,
 	) { }
 
 	public execute(): void {