Replace broken open-coding of FromStr with derive(clap::ValueEnum)

[MarijnS95]

Fixes #194, closes #196, CC @SolidTux for testing.

We've seen that impl FromStr for Format missed the "exe" match arm in parsing (#194) despite being listed in the allowed values in our top-level documentation and help text.

Instead of fixing this mistake, remove open-coded FromStr entirely and rely on clap's excellent ValueEnum derive. This not only generates a complete parser (with the option for attributes to change variant names) but also automates generation of a list of possible values, getting rid of any ambiguity/duplication/copy-paste errors we might have or create in the future:

❯ cargo r -- build -h
...
Usage: x build [OPTIONS]

Options:
      ...
      --platform <PLATFORM>
          Build artifacts for target platform [possible values: android, ios, linux, macos, windows]
      --arch <ARCH>
          Build artifacts for target arch [possible values: arm64, x64]
      ...
      --format <FORMAT>
          Build artifacts with format [possible values: aab, apk, appbundle, appdir, appimage, dmg, exe, ipa, msix]
      --store <STORE>
          Build artifacts for target app store [possible values: apple, microsoft, play, sideload]

Finally, we also utilize its generated PossibleValue::get_name() to implement our open-coded Display implementations, ensuring any renames via clap attributes trickle through into how we print them.