* Tag trim operator as a string operator
With the new functionality of the trim operator in 5.1.23, it's now a
general-purpose string manipulation tool. So it should be given the
"String operators" tag so that people see it in the same list as
removeprefix and removesuffix, etc.
* Document suffix of trim operator
* 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.
