fix(developer): prevent invalid values in targets store

[mcdurdin]

Fixes: #11773

[keymanapp-test-bot[bot]]

User Test Results

Test specification and instructions

User tests are not required

Test Artifacts

• Developer
  • Keyman Developer
  • Compiler Regression Tests
  • kmcomp.zip
• Keyboards
  • Test Keyboards

[DavidLRowe]

Not super important, but if you're making changes in this area anyway... Would you consider adding a warning for cases that are not wrong, but perhaps confusing? I'm thinking primarily of:

• any windows macos linux web ... (where the user has checked all the boxes)

but maybe "any" plus another box would also fall in that category.

[mcdurdin]

@DavidLRowe I've created #11996 for this but pushed it to a future release because I have no capacity to do it at present.

[keyman-server]

Changes in this pull request will be available for download in Keyman version 18.0.73-alpha