Automate installation of clangd and start language server using it

[dschaefer]

One of the key missions of this extension is to provide a user friendly way to get clangd onto the developer's machine. We could bundle it, and the clang-headers, for all three major platforms. Or we could download it from a server.

We would then startup the language server for it, becoming another client that integrates clangd.

The main advantage of this one is that we support non-clang environments where the user may not even have clang, and thus clangd installed. Most embedded environments are currently like that and use gcc instead.

[dschaefer]

This relies a bit on what we're going to do with clangd. I still feel we need to fork it, if anything, to add support for gcc compilers that are only being added for clangd and probably shouldn't be pushed upstream, unless that's a common thing.