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/renderDocument.ts

@@ -4,14 +4,14 @@
  *--------------------------------------------------------------------------------------------*/
 
 import { Command } from '../commandManager';
-import { MarkdownEngine } from '../markdownEngine';
+import { MarkdownItEngine } from '../markdownEngine';
 import { SkinnyTextDocument } from '../workspaceContents';
 
 export class RenderDocument implements Command {
 	public readonly id = 'markdown.api.render';
 
 	public constructor(
-		private readonly engine: MarkdownEngine
+		private readonly engine: MarkdownItEngine
 	) { }
 
 	public async execute(document: SkinnyTextDocument | string): Promise<string> {