chore: revive maintenance release

[iexitdev]

Summary

Revives the library for a maintenance release as react-native-chart-kit@6.12.1. This focuses on restoring reliable package builds, fixing low-risk runtime issues, and adding baseline CI.

What Changed

Build and Release Hygiene

• Bumped package version from 6.11.0 to 6.12.1.
• Updated TypeScript to ^5.4.5.
• Added typecheck.
• Updated prepare to use npm run build.
• Added skipLibCheck.
• Pinned/overrode @types/react to 16.14.8.
• Verified package contents with npm pack --dry-run.

CI

• Added GitHub Actions workflow for PRs and master.
• CI installs with npm install --legacy-peer-deps --ignore-scripts.
• CI runs npm run build.

Runtime Fixes

• Replaced unstable Math.random() React keys with deterministic keys.
• Added per-chart SVG definition IDs to avoid gradient collisions across multiple chart instances.
• Updated chart components to use scoped gradient URL helpers.
• Guarded dataset shadow gradient rendering when no dataset data is present.
• Wrapped custom line-chart dot content in keyed fragments.

Contribution Graph

• Replaced UNSAFE_componentWillReceiveProps with componentDidUpdate.
• Preserved existing update behavior when values changes.
• Fixed the onDayPress callback type.

Package Entry Points

• Fixed stale root imports in index.js so they match actual source filenames and casing.

Docs

• Added 6.12.1 changelog notes.
• Added branch naming conventions.
• Updated contributor setup/build commands to npm.

Validation

npm run build
npm run typecheck
npm_config_cache=/tmp/rnck-npm-cache npm pack --dry-run