Option for shift to switch to the first popup key for non-alphabetic keys

[Vftdan]

Is your feature request related to a problem? Please describe.
Additional punctuation keys are not influenced by "shift" key.

Describe the solution you'd like
Shift key should swap main key with its first popup key if its character is case-unaware.
Optionally, this should not work when in shift-locked state to imitate PC CapsLock behavior.

Use case
I use language layouts with builtin number row and additional punctuation keys. I want to be able to type punctuation like exclamation point, curly braces, question mark by sequentiallypressing shift and one/square bracket/slash respectively.
This is a generalization of this issue: #558

Describe alternatives you've considered (if any)
Using shift_state_selector: this requires switching from simple syntax to json syntax and copy-pasting of remaining popup keys for each numeric/punctuation key.