Add GitHub Actions CI pipeline

[cscully-allison]

Run build + tests on every push to main and every PR across Ubuntu/macOS/Windows x Python 3.10-3.12:

• npm ci, build esbuild bundles, run mocha
• pip install + pytest
• build the wheel, install it into a clean venv, and assert the compiled JS bundle physically shipped (catches packaging regressions)

Supporting fixes surfaced while wiring this up:

• setup.py: package_data only declared 'guidepost.js', so the wheel shipped without the static/.js bundles the widget loads at runtime. Include static/.js and *.js.map (+ MANIFEST.in for the sdist).
• .mocharc.js: was empty/untracked, so bare npx mocha discovered no tests; point it at tests/**/*.test.js.
• tests/data/polaris_sample.csv.gz: committed ~1MB sample (header + first 20k rows of the real POLARIS export) so the LOCATION-parsing regression runs in CI instead of skipping. Large real datasets are gitignored.