feat(ios): match iOS stock keyboard backspace key-cap behavior

### Describe the bug

iOS backspace, toggle between Outline to Solid filled glyph.

Normal state, outline glyph is presented.
When the key is pressed, solid filled glyph is presented, until the key is released.

https://github.com/keymanapp/keyman/assets/14062593/3a0c3ec4-b882-4c61-a814-0393236bdae5


### Reproduce the bug

_No response_

### Expected behavior

_No response_

### Related issues

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### Keyman apps

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

### Keyman version

_No response_

### Operating system

_No response_

### Device

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### Target application

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### Browser

_No response_

### Keyboard name

_No response_

### Keyboard version

_No response_

### Language name

_No response_

### Additional context

_No response_