Allow execution as `venvstacks`

[ncoghlan]

The project currently only supports execution as python -m venvstacks, since it seemed odd to provide a direct execution interface in a project that strips the direct execution scripts for all of the packages that it republishes.

However, that rationale doesn't actually hold:

• venvstacks explicitly aims to produce the same output artifacts regardless of the Python version used to run the stack build process (so tight control of the Python version used for execution doesn't matter that much)
• leaving out the entry point wrapper scripts means that Python utility installation and execution commands like pipx install venvstacks, uv tool install venvstacks, and uvx venvstacks don't work

This issue covers:

• adding the relevant entry point definition to invoke venvstacks.cli:main
• adding a new test case that ensures venvstacks --help succeeds

It's OK if the help text output changes to always report venvstacks as the invocation method, even when invoked as python -m venvstacks or python -m venvstacks.cli (although setting it appropriately based on __main__.__spec__ would be a nicer option)

[ncoghlan]

Local testing on the PR branch:

~/devel/venvstacks$ pipx install .
  installed package venvstacks 0.1, installed using Python 3.12.6
  These apps are now globally available
    - venvstacks
done! ✨ 🌟 ✨

~/devel/venvstacks$ venvstacks --help

 Usage: venvstacks [OPTIONS] COMMAND [ARGS]...

 Lock, build, and publish Python virtual environment stacks.

╭─ Options ───────────────────────────────────────────────────────────────────────╮
│ --help          Show this message and exit.                                     │
╰─────────────────────────────────────────────────────────────────────────────────╯
╭─ Commands ──────────────────────────────────────────────────────────────────────╮
│ build          Build (/lock/publish) Python virtual environment stacks.         │
│ local-export   Export layer environments for Python virtual environment stacks. │
│ lock           Lock layer requirements for Python virtual environment stacks.   │
│ publish        Publish layer archives for Python virtual environment stacks.    │
╰─────────────────────────────────────────────────────────────────────────────────╯