feat: add reference to back_1 from keymanweb-osk.ttf

### Is your feature request related to a problem? Please describe.

Need reference for special character
back_1 : 0x21
in
web/src/engine/osk/src/specialCharacters.ts


This glyph would be needed in creating iOS stock-keyboard like keyman-keyboards.


### Describe the solution you'd like

add in
'*BkSp1*': 0x21,
in
web/src/engine/osk/src/specialCharacters.ts

### Describe alternatives you've considered

_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
- [X] 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_