Integrate vscode-r-lsp - #695

Tested on WSL (Ubuntu) and everything seems to work nicely as before. No errors in dev console or output channel either

@randy3k @Ikuyadeu Do you think we could merge this PR before the next release to the marketplace?

We haven't released it since I moved from Ikuyadeu.
I think that there is no problem once we check the display of the Marketplace.

How are we going to deprecate vscode-r-lsp?

How are we going to deprecate vscode-r-lsp?

Now before vscode-R starts the language server, it checks if vscode-r-lsp is enabled. If so, vscode-R does not start the language server and advice the user to uninstall vscode-r-lsp.

Then I'll add a visible note to the README of vscode-r-lsp saying that the extension has been merged into vscode-R and will be unpublished at some point. Some time later, we could officially unpublish vscode-r-lsp and no user could search or install it from within vscode extensions panel.