Allow for comma-separated KeyVal arguments

[dginev]

This PR is a patch follow-up to #2683 . Once I started testing I realized we did not have the right infrastructure in place to recognize and preserve comma-separated values in KeyVals that enter through package options, and in particular via \usepackage.

• Adjusts the wrongly written conditional in \lx@ifusebbl from my previous PR
• Enhanced TrimmedCommaList to respect inner wrapping braces, as well as to trim outer wrapping braces.
• Moved TrimmedCommaList to KeyVal, as it should be in the Core layer and should in theory be useful in value processing.
• I also added tests to ensure I am really seeing the expected behavior this time.

There is likely a better patch out there, and I am happy to pursue it. The current PR allows for a minimal stability path so that we can use the new bibconfig={bbl,bib} recipe for arXiv.

[dginev]

As it turns out, the changes in the PR so far are not yet enough to deal with

--preload=[bibconfig={bbl,bib}]latexml.sty

But they suffice for the \usepackage variation. I will see if there is an easy extra commit to get preloads working as well.

[dginev]

I think I now have --preload working as well, again via TrimmedCommaList as the single utility sub.

Another detail is that to pass the braces as literal characters in a linux shell, they need to be escaped. So my test example was ultimately:

latexmlc test.tex  --preload=[bibconfig=\{bib,bbl\}]latexml.sty