feat(developer): &CasedKeys does not work with otherwise rule-less keys

**Describe the bug**
&CasedKeys doesn't work with keys that haven't been defined and therefore produce the US English default. (Perhaps this is how it is designed, in which case this should be a feature request!)

**To Reproduce**
I encountered this when reviewing https://github.com/keymanapp/keyboards/pull/1519 and suggesting the author add: `store(&CasedKeys) 'a'..'b' 'd'..'g' 'i'..'p' 'r'..'w' 'y'..'z'` to the keyboard to apply CapsLock logic to keys that were not redefined. When I added the statement, I found I needed to add: `+ 'a' > 'a'` and `+ 'A' > 'A'` to get CapsLock to work with the "a" key.

**Expected behavior**
I expected that putting 'a' in the &CasedKeys store would apply the CapsLock logic to the existing default rules for 'a' and 'A'.

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**Keyman for Windows:**
- OS: Windows 10 20H2 x64
- Keyman version: 14.0.271

**Keyman Developer:**
- OS: Windows 10 20H2 x64
- Keyman Developer version: 14.0.271.0