readline: text manipulation does not handle characters comprised of two UTF-16 code units

[Avi-D-coder]

• Version: v11.7.0
• Platform: Linux hostname 4.20.3-arch1-1-ARCH #1 SMP PREEMPT x86_64 GNU/Linux

readline backspace and delete do not handle astral characters correctly. If the cursor is within one char of the non UTF-16 char then delete, backspace, and or inserting a letter may not function correctly. Specifically they may delete one of the two code points, act on the astral char when they should be acting upon its neighbor, or insert between the two codepoints comprising an astral character.

[Avi-D-coder]

Found the problem. I will submit a PR.