FR: check for ccls in PATH, if not ask for setup and ideally auto-download

[GitMensch]

Observed behavior

Starting vscode on a "clean" machine, then installing the extension leads to "doesn't work" - because ccls is not installed.

Expected behavior

A nice error message with an option to downlaod ccls - and if user says yes do so to globalStorage and setup ccls.launch.command accordingly.

This is what the clangd extension does, result:

   "clangd.path": "c:\\Users\\me\\AppData\\Roaming\\VSCodium\\User\\globalStorage\\llvm-vs-code-extensions.vscode-clangd\\install\\16.0.2\\clangd_16.0.2\\bin\\clangd.exe",