Provide a UI element to refresh interpreter discovery via New Session UI

[jthomasmock]

UI companion issue to: #4269

Currently, the way to find newly created interpreters is to go to the Command Palette -> Python: Select Interpreter -> click small 🔄 in top right. It would be nice to surface this in a more obvious way; perhaps as its own Command Palette line or in the interpreter selector UI.

Note: some interpreters can be auto-found, eg, venv, conda, when created from the Create new environment option in the command palette.