For Keyman Developer Server, add app.webmanifest so that it can be saved
to the home screen; other pages should not require this.
Fixes: #16172
Test-bot: skip
- replace use of `kmw` with `keyman`
- unify pattern to load keyboards: do it in `keyman.init().then()` instead
of in the document `onload`
Part-of: keymanapp/keyman.com#449
Test-bot: skip