Removed use of `tabs.executeScript()` and updated manifest files

[tdulcet]

• Removed use of tabs.executeScript() (required for MV3)
• Updated manifest files:
  • Removed use of browser_style: true (required for MV3, see: https://github.com/TinyWebEx/CommonCss/issues/4)
  • Use background scripts instead of a background page (requires Firefox/Thunderbird 112 or greater)
• Updated ESLint version:
  • Made minor ESLint fixes enabling additional rules (see: https://github.com/TinyWebEx/AddonTemplate/issues/6).
• Backported changes to content script from https://github.com/rugk/awesome-emoji-picker/pull/93/commits/2cf1237160cc1d8cbaf759f6bc70f53fca84b6cb

[tdulcet]

Yes, I tested it and everything seems to work as before.