feat(markdown): add render command (fixes #75612)

This adds a command which renders the provided document, or the active
editor if one is provided. Following the pattern of some of the preview
commands, it returned `undefined` if there's no document provided and
no active text editor. Otherwise, seems to work...

```ts
const html = await vscode.commands.executeCommand<string>('markdown.render');
```

A way to render arbitrary strings in addition to documents may be useful at
some point in the future. However, I didn't implement that here as that'd
require some refactoring of the markdown engine. If we're interested though
I could certainly give that a shot.

extensions/markdown-language-features/package.json

@@ -24,6 +24,7 @@
 		"onCommand:markdown.showLockedPreviewToSide",
 		"onCommand:markdown.showSource",
 		"onCommand:markdown.showPreviewSecuritySelector",
+		"onCommand:markdown.render",
 		"onWebviewPanel:markdown.preview"
 	],
 	"contributes": {
@@ -325,4 +326,4 @@
 		"webpack": "^4.1.0",
 		"webpack-cli": "^2.0.10"
 	}
-}
+}