community/texlive: split out libsynctex

closes #13378 (closed)

installing texlive does pull this in as a subpackage:

< doas apk add texlive
(1/6) Installing texmf-dist (2021.58710-r2)
(2/6) Installing libpaper (1.1.28-r1)
(3/6) Installing libpaper-doc (1.1.28-r1)
(4/6) Installing libsynctex (20210325-r6)
(5/6) Installing texlive (20210325-r6)
(6/6) Installing texlive-doc (20210325-r6)

so i don't think normal use of texlive would be affected.
for reference, the arch package contains only this, and the manpages (dev files are still best left in -dev here): https://archlinux.org/packages/extra/x86_64/libsynctex/files/ . our docs are split, so i guess it's fine to leave them in texlive-doc
@maribu let me know if this is the correct way to split it- and see the attached issue for reasoning