bug(web): 'kmw.beforekeyboardchange' return type ignored

### Describe the bug

Digging into KMW's keyboard-activation code, I noticed some issues about this event:

https://help.keyman.com/DEVELOPER/ENGINE/WEB/15.0/reference/events/kmw.beforekeyboardchange

Note the bit under "Return Value"...

> ### Return Value
> `boolean`
> - `true` if the event should continue processing, `false` if it should not. Your event handler should return `true` aside from exceptional circumstances.

KMW currently ignores it entirely, even before modularization. In fact, it's been this way since at least KMW 2.0:

https://github.com/keymanapp/keyman/blob/75864966f480c2773980faefe3d35021165d6d53/web/source/keymanweb.js#L3122-L3124

https://github.com/keymanapp/keyman/blob/75864966f480c2773980faefe3d35021165d6d53/web/source/keymanweb.js#L3210-L3216

We'd need to condition on the `doBefore...` statements for there to be any effect for `false`. As such, a return value of `false` will not take effect to block a change of keyboard.

----

In discussion with others...

> Let's create an issue to track the bug. Then we can fix it in 17.0, and update the documentation accordingly when ready.
>
> Assuming it's possible to return false and block the change!

This is very much possible for the 'legacy event' type KMW has long supported, based on my analysis of the code.

### Reproduce the bug

_No response_

### Expected behavior

_No response_

### Related issues

_No response_

### Keyman apps

- [ ] Keyman for Android
- [ ] Keyman for iPhone and iPad
- [ ] Keyman for Linux
- [ ] Keyman for macOS
- [ ] Keyman for Windows
- [ ] Keyman Developer
- [ ] KeymanWeb
- [ ] Other - give details at bottom of form

### Keyman version

_No response_

### Operating system

_No response_

### Device

_No response_

### Target application

_No response_

### Browser

_No response_

### Keyboard name

_No response_

### Keyboard version

_No response_

### Language name

_No response_

### Additional context

_No response_