[HTML] Auto tag-closing doesn't work with undo/redo and programmatic changes

We had a user [report an issue](https://github.com/MicrosoftDocs/live-share/issues/610) with Visual Studio Live Share, where their HTML tag-closing behavior would insert duplicate closing tags as soon as a guest connected to their session.  After looking into it briefly, it appears like the [HTML tag-closing behavior](https://github.com/Microsoft/vscode/blob/master/extensions/html-language-features/client/src/tagClosing.ts) is [listening to the `workspace.onDidChangeTextDocument` event](https://github.com/Microsoft/vscode/blob/master/extensions/html-language-features/client/src/tagClosing.ts#L12:12), and therefore, generating the closing tag on every participants machine, as soon as the `/` or `>` is synchronized across the session. If five guests joined the session, then you'd see six copies of the closing tag. It isn't immediately clear what the best fix for this is, so I was hoping to get some advice. Initially, I thought we could set the `html.autoClosingTags` setting to `false` in the guest's workspace file. However, since the tag-closing behavior [relies on the `activeEditor`](https://github.com/Microsoft/vscode/blob/master/extensions/html-language-features/client/src/tagClosing.ts#L58), this would break tag-closing for guests if they weren't focused on the same file as the host (maybe that's a better short-term solution?). At a high-level, it seems like there are two possible ways to address to: 1. **Run the tag-closing logic on the host** - Live Share already takes "exclusive" ownership of language services for `vsls:` documents, but the tag-closing logic runs on the guest's machine since it appears to be [always initialized](https://github.com/Microsoft/vscode/blob/master/extensions/html-language-features/client/src/htmlMain.ts#L90) regardless of the document scheme. We could explore ways to run this logic on the host, however, the reli…