Migrate default themes to TypeScript

[JezerM]

Related to #13

Changes

• Gruvbox and Dracula themes migrated to TypeScript.
  • Added new mock.ts file to both themes
  • Use JavaScript modules.
• Fix some bugs.
  • Removed transition CSS rule for all elements in HTML for Dracula, which improves performance.
  • Fix localStorage issue when saving/loading the default user.
• Added more NPM scripts:
  • Build themes with npm run build:themes
  • Lint themes with npm run lint:themes, and fix with npm run lint:themes:fix
  • Prettier themes with npm run prettier:themes, and fix with npm run prettier:themes:fix
  • npm run build now builds both themes and nody-greeter.