feature: add support for Visual Studio as external tool

[czarkoff]

This allows using Visual Studio as an external tool for opening the repo in.

Current limitations:

• Visual Studio for Mac is not supported.
• It only relies on registry for Visual Studio location (reportedly Visual Studio can be installed without modifying the registry; other discovery mechanisms may be used).
• SourceGit tries to open the whole repo as a directory. From my experience, when you actually need Visual Studio (as opposed to Visual Studio Code), you normally want to open a solution file instead. One may expect a prompt asking how to open a project and allowing to select a solution file interactively. Instead, the implementation tries to locate a solution file and to open it directly, falling back to repo directory only in case of failure.

The solution finding bit is more objectionable, and the implementation is messier, so it is split out into its own commit.

Basically, I've implemented specifically what I need to make use of the external tools functionality.