Offer to set Ninja as preferred generator when present

This change (to use Visual Studio as a generator instead of Ninja when compiling with MSVC) has broken my build environment :(. This was is a breaking change but doesn't appear to have been treated as such.

This was also previously advertised as a _feature_ of this extension, per the documentation linked in the extension's homepage.

https://vector-of-bool.github.io/docs/vscode-cmake-tools/kits.html#kits

Quote:
> If you use Ninja there is no need to worry about Visual Studio CMake Generators. CMake Tools will prefer Ninja if it is present unless configured otherwise.

_Originally posted by @skalldri in https://github.com/microsoft/vscode-cmake-tools/issues/1084#issuecomment-652182566_

Start by reviewing the linked kits documentation and the existing generator-selection behavior in the extension. Identify where the preferred generator is chosen and determine how an option to prefer Ninja when present should be exposed; done means MSVC users can opt into the documented Ninja preference without breaking the Visual Studio default.

Written by the indexing model from the issue text.