Closes#20476
Release Notes:
- Fixed a bug in toolchain selector that caused it to not pick up venvs
for tabs before user interacted with them.
- Fixed a bug in language selector that caused it to pick up Markdown as
the language for a buffer up until the tab was interacted with.
This PR adds support for selecting toolchains for a given language (e.g.
Rust toolchains or Python virtual environments) with support for SSH
projects provided out of the box. For Python we piggy-back off of
[PET](https://github.com/microsoft/python-environment-tools), a library
maintained by Microsoft.
Closes#16421Closes#7646
Release Notes:
- Added toolchain selector to the status bar (with initial support for
Python virtual environments)