Allow StringNameSpace::split() to take an expression as its first argument

[rben01]

Problem description

Current, StringNameSpace::split() only lets one split on a fixed string. It would be nice to be able to split on an expression that evaluates to a string. This way you could use a different separator per row.

In my case, I have lists-as-strings that use commas as the separator in some years and semicolons in other years, and to get the original list back, I'd like to be able to select the separator to turn these into a list on a year-by-year basis, but it doesn't seem possible.