Expose a way to trigger environment prompts for other extensions

[karrtikr]

Expose a command which triggers #19396. Can be used for:

• Restarting the tools server, say Black: Restart Server.
• Other manual triggers, for eg. when running a formatter manually.

cc/ @karthiknadig

[karrtikr]

Command to execute: python.triggerEnvSelection

If an invalid interpreter is selected:

If no Python is available to select: