feat(simplexpr): add uppercase and lowercase functions

[DOD-101]

Description

Add two functions uppercase(string) & lowercase(string) to convert strings.

Usage

  (label :text "${uppercase("word")}")
  (label :text "${lowercase("Word")}")
   
   (label
      :class "key-label"
      :text { osk_shift ? "${shift_letter ?: uppercase(letter)}" : "${letter}"}
   )

Showcase

Additional Notes

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing