* Support case insensitive matching in prefix/suffix operators
Support `caseinsensitive`/`caseinsensitive` suffixes in the following
filter operators:
* prefix
* suffix
* removeprefix
* removesuffix
The suffixes have the same behaviour as in the match operator.
Closes: #6407
* Do not filter titles if suffix/removesuffix operand is empty
Issue: #6407
* Add suffix and parameter to trim operator
Fixes#4809
* Unit tests for new trim operator parameters
* Mention trim operator in 5.1.23 release notes
* Address review comments
* Move regex escaping into utils.js trim functions
This way the trimPrefix and trimSuffix functions from utils.js are safe
to call without regex-escaping their parameters, which should make them
easier to use from other parts of the Javascript code.
