Python environments API

[karrtikr]

Definitions file: https://github.com/microsoft/vscode-python/blob/proposedenvapis/src/client/proposedApiTypes.ts

Example usage: https://github.com/microsoft/vscode-python/wiki/Proposed-Environment-APIs

Feedback being captured here: microsoft/vscode-python#18888

[karrtikr]

API is now finalized and going out in the next release of the Python extension, see microsoft/vscode-python#19886 for the test plan item.

[karrtikr]

Pending discussion of naming of known API.

[karrtikr]

Looks good from VSCode's end. known indicates there can be unknown environments in the system which are still to be discovered, so it seems like the best fit.