Refactor markdown language features (#152402)

(sorry for the size of this PR)

This change cleans up the markdown language features by making the following changes:

- Use `registerXSupport` public functions to register these
- Expose the slugifier the `MarkdownEngine` uses. You never want to use a different one if you already have a markdown engine
- Sort of clean up names. I'd introduced a bunch of confusing names while iterating in this space. What I'm working towards:

    - `Computer` — Stateless thing that computer data
    - `Provider` — Potentially stateful thing that provides data (which may be cached)
    - `VsCodeProvider` — The actual implementation of the various vscode language features (which should only be used by VS Code and in tests, not shared with other features)
- Introduce `MdLinkProvider` to avoid recomputing links for a given document. Also use this to hide more internals of link computation

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

@@ -98,10 +98,14 @@ export class MarkdownEngine {
 	private _slugCount = new Map<string, number>();
 	private _tokenCache = new TokenCache();
 
+	public readonly slugifier: Slugifier;
+
 	public constructor(
 		private readonly contributionProvider: MarkdownContributionProvider,
-		private readonly slugifier: Slugifier,
+		slugifier: Slugifier,
 	) {
+		this.slugifier = slugifier;
+
 		contributionProvider.onContributionsChanged(() => {
 			// Markdown plugin contributions may have changed
 			this.md = undefined;