Add "Default" language server setting

[jakebailey]

"Default" is the new default LS setting, and at runtime is transparently converted to Jedi/JediLSP/Pylance depending on the current state. This allows for Pylance to be used by default if it's installed, without any configuration. The existing settings allow for manual configuration.

This also makes it more obvious in the UI the behavior that will happen (as opposed to having the default say one thing, but then in reality it's using something completely different).

With #16116.

[luabud]

@jakebailey can we close this? 😊

[jakebailey]

Yes, sorry.

We may need to make a new issue for microsoft/pylance-release#1263 (comment) (or transfer it), though.