build: strip background.scripts for Chromium Manifest V3 support

[hkzo]

Summary

This PR introduces a build-time transformation via build.py to generate a Manifest V3-compliant manifest.json, allowing the extension to be compatible with modern Chrome/Chromium environments. This is motivated by the fact that warnings are now appearing in Chromium-based browsers due to the current manifest structure.
Following the comments in PR #4297, this build-time approach was chosen to avoid direct modifications to the source manifest.json.

[Manage Extensions]

Changes

Enhanced build.py: When targeting Chromium-based browsers, the script now automatically removes the background.scripts field from the manifest to comply with Manifest V3 requirements.
This change does not affect the Firefox build process.

Verification Results

• Confirmed that the generated manifest.json for Chrome does not contain background.scripts.
• Verified that the process also works correctly for Firefox.
• Confirmed that the warnings in Chrome have disappeared.

Notes

• The Chrome and Edge store packages I checked already have scripts removed and contain only service_worker. Checked package versions: Chrome 4.2081, Edge 4.2028, Firefox 4.2028.
• I used AI assistance for translating this PR description.