qmlls: implement support for VS (non-code)

It seems that the VS team can't find a way to run qmlls per-project, the VS API seems to only allow running language server globally across a Solution. All projects inside a solution are sharing the same language server.

This is not really good for qmlls, because it won't get the command line parameters it needs (that are per-project).

The required command line parameters are currently:

• import paths
• documentation folder
• build directory

QTBUG-134308 should take care of separating the project-specific information, like build directories, between the different workspace folders.

QTBUG-134307 should take care of import paths and documentation folder.

Once QTBUG-134308 and QTBUG-134307 are done, extend the LSP protocol to be able to communicate the build folder of a workspacefolder to qmlls.