#859 \uniqueyear for custom export filters

[attractedtofire]

Hello everyone!

I am writing this in addition to a bug ticket I just submitted (#1218 regex grouping not working as expected).

Working on custom export filters I keep running into the need of having "unique years", that is years+abcd.. : Mayer2001a, Mayer2001b.
One obvious solution would be to simply use the second half of the bibtex-key (something like \format[Replace("^\w+(?=\d+[a-z]$),")]{/bibtexkey}) but already this simple case is not exactly helping the read- and maintainability of the filters and this rather quickly gets more complicated (having to deal with more unusual author names, non-number-values for year like "in press", "forthcomming",... etc.) and/or when the need for custom bibtexkeys arises.

As this, at least in the fields I am concerned with, is a relatively common need, I think having direct access to a "unique year" would be very useful indeed.

All the very best,
a