Make Stone easier to invoke locally

[aaronsky]

Description

Stacked on #359.

Allows stone/cli.py to bootstrap its package path when run directly and adds stone/__main__.py for module invocation.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

Verification run directly with the CI lint/test steps:

• uv run --no-project --with-requirements requirements.txt --with-requirements test/requirements.txt --with flake8==5.0.4 --with pylint flake8 setup.py example stone test
• uv run --no-project --with-requirements requirements.txt --with-requirements test/requirements.txt --with flake8==5.0.4 --with pylint --with setuptools pylint --rcfile=.pylintrc setup.py example stone test
• uv run --no-project --with-requirements requirements.txt --with-requirements test/requirements.txt --with pytest --with mock pytest
• uv run --no-project --with-requirements requirements.txt --with-requirements test/requirements.txt --with enum34 --with mypy --with types-six ./mypy-run.sh

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.30%. Comparing base (1ad079b) to head (01e9971).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
stone/__main__.py	0.00%	3 Missing ⚠️
stone/cli.py	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
+ Coverage   49.98%   51.30%   +1.31%     
==========================================
  Files          37       38       +1     
  Lines        8808     8876      +68     
  Branches     1892     1913      +21     
==========================================
+ Hits         4403     4554     +151     
+ Misses       4087     3979     -108     
- Partials      318      343      +25     

Flag	Coverage Δ
unit	51.30% <0.00%> (+1.31%)	⬆️

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