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/languageFeatures/pathCompletions.ts

@@ -5,7 +5,7 @@
 
 import { dirname, resolve } from 'path';
 import * as vscode from 'vscode';
-import { MarkdownEngine } from '../markdownEngine';
+import { IMdParser } from '../markdownEngine';
 import { TableOfContents } from '../tableOfContents';
 import { resolveUriToMarkdownFile } from '../util/openDocumentLink';
 import { SkinnyTextDocument } from '../workspaceContents';
@@ -82,7 +82,7 @@ function tryDecodeUriComponent(str: string): string {
 export class MdVsCodePathCompletionProvider implements vscode.CompletionItemProvider {
 
 	constructor(
-		private readonly engine: MarkdownEngine,
+		private readonly parser: IMdParser,
 		private readonly linkProvider: MdLinkProvider,
 	) { }
 
@@ -249,7 +249,7 @@ export class MdVsCodePathCompletionProvider implements vscode.CompletionItemProv
 	}
 
 	private async *provideHeaderSuggestions(document: SkinnyTextDocument, position: vscode.Position, context: CompletionContext, insertionRange: vscode.Range): AsyncIterable<vscode.CompletionItem> {
-		const toc = await TableOfContents.createForDocumentOrNotebook(this.engine, document);
+		const toc = await TableOfContents.createForDocumentOrNotebook(this.parser, document);
 		for (const entry of toc.entries) {
 			const replacementRange = new vscode.Range(insertionRange.start, position.translate({ characterDelta: context.linkSuffix.length }));
 			yield {
@@ -349,8 +349,8 @@ export class MdVsCodePathCompletionProvider implements vscode.CompletionItemProv
 
 export function registerPathCompletionSupport(
 	selector: vscode.DocumentSelector,
-	engine: MarkdownEngine,
+	parser: IMdParser,
 	linkProvider: MdLinkProvider,
 ): vscode.Disposable {
-	return vscode.languages.registerCompletionItemProvider(selector, new MdVsCodePathCompletionProvider(engine, linkProvider), '.', '/', '#');
+	return vscode.languages.registerCompletionItemProvider(selector, new MdVsCodePathCompletionProvider(parser, linkProvider), '.', '/', '#');
 }