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: Remove dependency on browser_style: true TinyWebEx/CommonCss#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: Consider adding more ESLint rules TinyWebEx/AddonTemplate#6).

[rugk]

LGTM, this is tested to still work?

[tdulcet]

Yes, I tested it and everything seems to work as before. Note that some minor CSS adjustments may be needed in your library: TinyWebEx/CommonCss#4. For example, I now see some "Times New Roman" font in the popup.