feat: simplify game-bridge build using native BigInt support

Remove unnecessary polyfills and transformations by leveraging
UnityWebBrowser 2.2.7 (CEF 137 = Chrome 137) native BigInt support.

Changes:
- Remove JSBI, whatwg-fetch, Babel polyfills from game-bridge
- Replace transform-bigint.js with fixUnityBuild.js
- Inline all JS into single script block without code transformations
- Update Parcel target to Chrome 137 (current CEF version)
- Add Parcel bundler configuration (.parcelrc)
- Modify index.html to use external module reference

This approach eliminates regex-based code transformations that were
corrupting string literals and causing syntax errors.

Author: rodrigo-fournier-immutable