breaking: Implement new API approach that has shape `extension SUB_COMMAND`

[schloerke]

Fixes #23 Fixes #24 Fixes #25 (untested)

TODO

• [x] Ask python --version to see if high enough for smoother transition.

TODO - On approval:

• [x] Tag prev commit with v1
• [x] Merge https://github.com/posit-dev/py-shinylive/pull/16
• [ ] Merge https://github.com/posit-dev/r-shinylive/pull/21