Text: Make backspace delete last code point.

This implements last code point removal for backspace, to fix handling deletion of conjuncts in indic scripts.

This also fixes the text removal command and has special code to ensure deletion doesn't end in a grapheme ending with ZWJ (as that merges graphemes) as well as handling variation selectors and regional codes for flags.

Test Plan

• Make a text with "क्रिता".
• press backspace
• see only last 'bar' dissappears.
• Delete will still remove the whole grapheme
• Note that if you compare to other implementions, you will need to check how many cursor positions are possible, Kate for example divides the sample text into two graphemes, while firefox and krita both consider it three. I suspect that when firefox and for us, libunibreak switch to unicode 15.1 they might consider it two segments as 15.1 implemented extra support for indic script. More details here

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.