feat: added tests, casks/tap support, dynamic versioning, theme persistence, brew tap release

[bhavya-dang]

Changelog

Closes #31 Closes #29 Closes #27

• Added comprehensive tests
• Casks and taps now show [c] or [t] identifiers besides their entries in the list (detail pane not populated as of now)
• Added goreleaser instructions to release new version of the app's tap on brew
• Theme will now be persisted in a $HOME/pkgui/config.json file where $HOME is OS-agnostic.
  • can be configured using the --config flag
• Cobra integration now allows help, version, and config flags. Run pkgui --help