feat: Automatically remove top-level MV2-only or MV3-only keys

[aklinker1]

This closes #517.

Instead of having to use a function to change the keys between manifest versions, you can just include both the MV2 and MV3 keys in a single object. When targetting MV2, all MV3 keys will be removed. When targetting MV3, all MV2 keys will be removed.

Before

export default defineConfig({
  manifest: ({ manifestVersion }) => {
    const manifest = {
      // ...
    };
    if (manifestVersion === 2) manifest.browser_action = {};
    else manifest.action = {};
    return manifest;
  };
});

After

export default defineConfig({
  manifest: {
    // ...,
    action: {},
    browser_action: {},
  },
});

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.53%. Comparing base (8a6acba) to head (d6432b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
+ Coverage   86.50%   86.53%   +0.02%     
==========================================
  Files         111      111              
  Lines        8798     8847      +49     
  Branches      867      871       +4     
==========================================
+ Hits         7611     7656      +45     
- Misses       1173     1177       +4     
  Partials       14       14              

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