chore(developer): make package subfile description fully optional

[mcdurdin]

The package subfile description field is never really used in the end-user apps. As it is meaningless metadata, we'll make it fully optional, and start to remove all references in the designers and compilers.

TODO: For packages targeting Keyman 18.0+, we should consider removing the description field altogether from the subfiles. This should be done in a separate PR.

@keymanapp-test-bot skip

[keymanapp-test-bot[bot]]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

• Android
  • Keyman for Android apk
  • FirstVoices for Android 16.0 apk
  • FirstVoices Keyboards for Android apk
  • KeyboardHarness apk
  • Keyman for Android 16.0 apk
  • KMSample1 apk
  • KMSample2 apk
• Developer
  • Keyman Developer
  • Compiler Regression Tests
  • kmcomp.zip
• iOS
  • Keyman for iOS (simulator image)
  • FirstVoices Keyboards for iOS (simulator image)
  • TestFlight internal PR build version - 18.0.142 (0.12665.12275)
• Keyboards
  • Test Keyboards
• Web
  • KeymanWeb Test Home
• Windows
  • Keyman for Windows
  • FirstVoices Keyboards for Windows
  • Text Editor (32 bit)
  • Text Editor (64 bit)

[mcdurdin]

TODO: For packages targeting Keyman 18.0+, we should consider removing the description field altogether from the subfiles. This should be done in a separate PR.

See #12682