bug(web): intermittent K_BKSP rule is deleting more than it should

```
c marc_maori_2 generated from template at 2023-07-14 14:29:39
c with name "marc_maori_2"
store(&VERSION) '15.0'
store(&NAME) 'marc_maori_2'
store(©RIGHT) '© Marc'
store(&KEYBOARDVERSION) '1.0'
store(&TARGETS) 'any'
store(&BITMAP) 'marc_maori_2.ico'
store(&VISUALKEYBOARD) 'marc_maori_2.kvks'
store(&LAYOUTFILE) 'marc_maori_2.keyman-touch-layout'

begin Unicode > use(main)

group(main) using keys

store(vowels) 'aeiouAEIOU'
store(vowelsOut) 'āēīōūĀĒĪŌŪ'

+ '/' > 'ˉ'
'ˉ' + any(vowels) > index(vowelsOut, 2)
'ˉ' + '/' > '/'
'ˉ' + 'k' > 'ḵ'
'ˉ' + 'K' > 'Ḵ'
'Ḵ' + [K_BKSP] > nul
'ḵ' + [K_BKSP] > nul
```

On web, if I type x/kBackspace, the `x` is deleted as well as the `ḵ`, which does not happen on other platforms.

Sadly, I cannot consistently reproduce this but wanted to capture this issue for future review. I am not sure what the sequence of events was that got into this mode. I switched to another keyboard and back and then bksp worked as expected. (Test within Keyman Developer web debugger)

KeymanWeb 16.0.138