feat: Support browser-specific manifest generation for Chrome and Firefox

[sun-yryr]

Summary

Fixes #1066

This PR implements browser-specific manifest generation to support both Chrome and Firefox with Manifest V3:

• Added manifest template system: Created manifest.template.json with browser-specific keys using the pattern __{browser}__{key} (e.g., __chrome__service_worker, __firefox__scripts)
• Implemented manifest transformer: Added scripts/transform-manifest.js that processes the template and generates browser-specific manifest files during build
• Updated npm scripts:
  • Modified webpack config to support browser-specific builds via BROWSER environment variable
  • Added separate build scripts for Chrome and Firefox (build:chrome-mv3, build:firefox-mv3)
  • mv3 means Manifest V3
  • Added build:all script to build both browsers
  • Output directories are now browser-specific (dist-chrome-mv3, dist-firefox-mv3)
• Updated CI: Modified CI workflow to use build:all instead of build
• Updated .gitignore: Added /dist-* pattern to ignore all browser-specific dist directories

Testing

Verified that the extension successfully fetches pull request lists on the following browsers:

• Floorp (12.7.0, Firefox 145.0.1)
• Arc (1.122.0 (71105), Chromium Engine Version 142.0.7444.176)

Notes

release.yml is not updated yet because I'm not familiar with the extension deployment process to Chrome Web Store and Firefox Add-ons.

After this PR is merged, please update release.yml before releasing the extension.

If helpful, here is a related reference: https://github.com/sun-yryr/prmonitor/commit/89abd46d42e6de8032b884a1101d447a408f04fd

[CLAassistant]

All committers have signed the CLA.