Make changing default version of VS explicit

[BretJohnson]

From Slack comment:

Also, I think I'd prefer if the default didn't change when running an alternate version.
That way, one can have a default version of VS used for most solutions (e.g. stable) and switch to an alternate version on an exception basis, w/o changing the default.
To achieve that, you could have say a --setDefault option, along with the others, that updates the default when VS is launched.
Also, optionally - this is more of a nice to have I think - you could store the default in the registry as the default program associated with .sln files.
That way, the command line and File Explorer are in sync with the "default" version of VS, reading/writing the same setting.

As vswhere uses mixed case for option names (-requiresAny) could stick with that convention for --setDefault. And say have a --clearDefault option to remove the default. Or go with the mono.options convention of default+ and default- to turn it on/off.

[kzu]

I like --default+ and --default- 👍