Allow saving the default VS to use for a specific directory and its descendents

[kzu]

We made it possible to select a default VS to run when vs run (or simply vs) is used.

It's not unusual to work on various solutions/repository roots with different versions of VS: for example, the main extension you work on always runs VS preview, while your OSS projects always use VS stable.

Proposal:

vs [run] +default=.

So the default flag instead of being a "plain" boolean like before (we had --default to set the default and --default- to remove the default) it would be a boolean parsed with + (to enable) and - to disable (just like the proposed syntax for modify) and the optional value would be the directory where to store the preference.

If a directory is not set (or . to denote current directory), the existing logic in dotnet-file to look up from the current directory until a .netconfig is found, or go to user profile would be used.

Given #10, it would now be possible to change the default VS for any folder and its descendents, since dotnet-file options reading will automatically probe for parent directories when looking for the devenv preference.