Bring back the a11y tree

[Tyriar]

We removed the old a11y tree in favor of the new "accessible buffer" in a7e0a9c, but we ended up moving the latter into vscode's codebase as it was getting too complicated to allow an embedder to implement this properly. Bringing this back may impact #4491

[Tyriar]

@meganrogge could you look at this some time next week? We can't release xterm.js without doing this.

It'll mostly just be reverting the commit, but we'll need to be mindful that in VS Code not to disturb the accessible buffer which must effectively replace this. We don't actually want it exposed to the user at all in VS Code.