feat: New @wxt-dev/is-background package

[aklinker1]

Overview

Pulled out of #1808 into a separate package so it's easier to test and use by other packages in the ecosystem.

• Test Chromium MV2 manually Not possible anymore
• Test Chromium MV3 manually
• Test Firefox MV2 manually
• Test Firefox MV3 manually
• Test Safari MV2 manually
• Test Safari MV3 manually
• Update docs
• Update CI
• Add co-author

Manual Testing

I have setup unit tests for each browser/manifest version combo, but this relies on my stubbed env being correct.

You could create a new extension and test it in all browsers manually.

Related Issue

Apart of #1807

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.28%. Comparing base (05e23e7) to head (2e08c0c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2152      +/-   ##
==========================================
+ Coverage   76.26%   76.28%   +0.01%     
==========================================
  Files         113      114       +1     
  Lines        3055     3065      +10     
  Branches      686      693       +7     
==========================================
+ Hits         2330     2338       +8     
- Misses        641      642       +1     
- Partials       84       85       +1     

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[github-actions]

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