[BUGFIX] Use multibyte method in CodeCompletionController

Use mb_strlen & mb_substr to cut off TypoScript to avoid exceptions
if the 20th character is a multibyte value.

Resolves: #90372
Releases: master, 9.5
Change-Id: I8f90aa54a4cb6ede95d4fa50d557aeec4d8cd031
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63255


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>