Add language toolchains (#19576)

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 #16421
Closes #7646

Release Notes:

- Added toolchain selector to the status bar (with initial support for
Python virtual environments)

assets/settings/default.json

@@ -779,6 +779,7 @@
   "tasks": {
     "variables": {}
   },
+  "toolchain": { "name": "default", "path": "default" },
   // An object whose keys are language names, and whose values
   // are arrays of filenames or extensions of files that should
   // use those languages.