added msbuild:UseBundledOnly option to force the usage of bundled MSBuild

[filipw]

At the moment there is no way to force bundled MSBuild on Windows.
On Linux/Mac it is possible in VS Code using omnisharp.useGlobalMono: never but it is a client specific option, not a universal server option for all clients.

What this PR introduces is the following setting to omnisharp.json

{
   "msbuild": {
       "useBundledOnly": true
   }
}

The effect is:

• on Windows, no Visual Studio / MSBuild Tools MSBuild is discovered anymore - only the bundled MSBuild is used
• on Linux/Mac, regardless whether running on global or local Mono - only the bundled MSBuild is used

This should unblock scenarios when:

• user has outdated Visual Studio
• user wants to run on global Mono but using the bundled MSBuild

Both of those have been repeated in multiple issues.