[FEATURE] Make MimicKit installable as a package

[Kashu7100]

1. pyproject.toml (new) — Added a pyproject.toml to make the project
   pip-installable via setuptools. Defines project metadata (name,
   version 0.1.0, license, authors), lists all dependencies (previously
   in requirements.txt), and configures package discovery to include
   mimickit*.
2. requirements.txt (deleted) — Removed in favor of the dependencies
   section in pyproject.toml.
3. mimickit/init.py — Added a sys.path hack that inserts the
   mimickit/ package directory onto sys.path. This preserves backward
   compatibility with the project's internal flat imports (e.g., import
   learning.ppo_agent) which expect submodules to be importable
   directly, regardless of whether the code is run as a script or
   imported from an installed package.

[xbpeng]

Thanks a lot for putting together this PR! This looks like it could be useful. But we probably won't be making this change for now.

But I think it would still be beneficial to keep this PR around, in case others might find it useful.