Upgrade to wxt v0.20+

[rthaut]

See: https://wxt.dev/guide/resources/upgrading.html#v0-19-0-rarr-v0-20-0

Initial takeaways and possible actions needed:

• Ensure builds work without the webextension-polyfill (or use the new @wxt-dev/webextension-polyfill)
  • TODO: need to switch to chrome global variable/namespace?
• New Browser type
• Move/configure public/ and modules/ directories
• Update wxt imports
• New default output directories