feat(developer): notify when .js is not included in the package when the keyboard supports touch/mobile

[MakaraSok]

Describe the bug

This is a good to have, but probably not of high priority.

According to the issue here, let's have Keyman Developer notify one that .js doesn't exist in the keyboard package while it supports mobile/touch devices.

[mcdurdin]

This is a nice suggestion. I will consider it for future work on the compiler.