Publish API as npm package

[eleanorjboyd]

The Python Environments Extension needs to release a first finalized (non-proposed) API package to npm. We are not ready to stabilize every part of the extensions API at this point, we want to still keep some items in proposed as we work to finalize them.

Given the following sections / distinctions we need to separate out the proposed parts from the current api file into a proposed-api.ts file to keep them distinct

Steps:

1. Separate out the API parts (finalized vs proposed) in the code
2. Publish Python Environments API to npm

Sections of the API ready to finalize:

• PythonEnvironmentManagerApi
• PythonPackageManagerApi

Sections of the API to stay as proposed:

• PythonEnvironmentVariablesApi
• PythonExecutionApi
• PythonProjectApi