New rewrite plan for extension manager

Fetch list

• Biscuit will clone the extension repo itself
• The extension repo contains submodules
• extensions.toml contains extension names mapped to their submodule name, name, version etc.

Installation

• Submodule of extension will be update --init'ed and that's all
• <extension_name>/extension.py will be entry point which must contain a def setup(api)
• and now, yes extensions can be using multiple files

Extension dev

• For now, a hello biscuit template has been added to github
• Later, this will be cloned when biscuit ext new ... command is used
• Maybe when there are more templates (language, theme, etc) we can go with biscuit ext new --<template> ...
• Extension dev runs -- something like biscuit ext dev will launch a biscuit instance with that extension on
• Extension tests -- I have added some tests to template repo demonstrating how it's done, so they can just run pytest and it will work

publishing and updating

• devs submodule the extension to extensions repo, and mark name, version etc. in the list
• for updates, the submodule is updated, version changed in list

Extension version control

• Since we're storing the version info within extension list, we can easily detect if anything is outdated (the list is fetched on every biscuit launch)
• upon user confirming update, just fetch, pull the submodule latest -- done!