Switch to ESM, modernize, upgrade deps

[mourner]

• Modernized to ESM: converted the package and all source/scripts/tests to ES modules, upgraded dependencies, and dropped legacy helpers.
• Fixtures as JSON: replaced all src/*.js fixture modules with plain src/*.json files, loaded at runtime.
• Test suite migration: ported tests from tape to the built-in node:test runner.
• Linting: added ESLint with eslint-config-mourner, configured for the codebase's 2-space indent, and fixed the resulting issues.
• Simpler layout: collapsed lib/generateBuffer.js into index.js, removed the lib/ directory, moved real-world-extents.json next to the scripts that use it, deleted the stale scripts/dump mason/protoc helper, and removed CHANGELOG.md (easier to maintain it as GitHub release notes).
• CI: refreshed the GitHub Actions workflow, added a lint step, and added a drift check that rebuilds fixtures/ and fails if anything changed — so the committed artifacts can't go stale silently.
• Package scripts: added lint and a prepublishOnly hook that runs lint + build + tests before publish.
• Docs/API: regenerated info.json files and synced README/API to match.

Note on -2 byte fixture changes

After the pbf dependency upgrade, regenerated tiles are 2 bytes smaller per feature. The previous version emitted an empty packed tags field (12 00) for features with no tags; the new version correctly omits the empty field, as expected for proto2 packed repeated fields. Decoded tile content is unchanged — only the redundant empty-field bytes are gone.

[branyip]

LGTM, thanks for the changes!