Fixes T13342. This does a few different things, although all of them seem small enough that I didn't bother splitting it up:
- Support export of "remarkup" custom fields as text. There's some argument here to export them in some kind of structure if the target is JSON, but it's hard for me to really imagine we'll live in a world some day where we really regret just exporting them as text.
- Support export of "date" custom fields as dates. This is easy except that I added null support.
- If you built PHP from source without "--enable-zip", as I did, you can hit the TODO in Excel exports about "ZipArchive". Since I had a reproduction case, test for "ZipArchive" and give the user a better error if it's missing.
- Add a setup check for the "zip" extension to try to avoid getting there in the first place. This is normally part of PHP so I believe users generally won't hit it, I just hit it because I built from source. See also T13232.