Support \U\u\L\l replace modifiers in global search/replace (see PR #96128)

[irridia]

Support \U\u\L\l modifiers to search replacement to alter word case in the global Find/Replace flow.

The behavior is patterned after Boost, but only applied to the replacement text, not subsequent content. Thus the \E isn't relevant. This is also vaguely similar to other replacement syntax, like pcre2 and perl.

For example, to export a file full of non-exported functions in a Go file:
Find:
^func (\w+)(
Replace:
func \u$1(
Example:
func myPrivateFunc(...
Result:
func MyPrivateFunc(...
With \U:
func MYPRIVATEFUNC(...
With \l\l\l\U:
func mypRIVATEFUNC(...

This PR broadens the implemetation as present in PR #96128.

@roblourens

[roblourens]

I need to spend some more time testing this but at first look it looks great!

[roblourens]

Thank you!