Add full type annotations for mypy

[CoolCat467]

In the vod for your stream, you mentioned potentially checking with mypy.
This pull request does exactly that, adding complete type annotations to allow for type checking with mypy as well as the necessary configuration file for mypy.

I have made two minor functional changes that are noteworthy:

1. Removing unused imports from typing module
2. Decoding shell command bytes as utf-8 in load_snapshots instead so return type of list[Snapshot] is still usable and we don't need to make a separate copy of Snapshot typed dictionary.

With mypy installed, program will be type checkable by running

mypy rere.py

in project root directory without any errors.