Add extra sanity checks in check_submission_dir(): it now checks towards the
beginning if the required files exist (that is, the three user submitted files,
not the JSON file as those don't exist yet, though it does check that they do
not exist as dot files here would be an error) and are readable and if they are
not size 0 (for Makefile and remarks.md - prog.c may be empty).

Updated some literal strings of the three required filenames (as above) to their
macros in both mkiocccentry and txzchk.

Fixes in chkentry with checking if files exist or do not exist (depending on
options) and also add checks for file sizes for certain files, namely Makefile,
remarks.md (in non-winning mode) as well as index.html (in winning mode) and
README.md (in winning mode).

Error messages were improved in chkentry as well though there are some places
(the manifest check) where they could be improved (this would require lower
level changes that we do not have time for before the freeze later today).

Sync [jparse repo](https://github.com/xexyl/jparse/) to jparse/ for new util
function (that uses its file_size() function) is_empty(). This is used in
the update to chkentry.

More work on the chkentry_test.sh script. The script is in better shape so
that when the directories are generated (they cannot be static due to version
changes - at least without a new option) the script should hopefully be easily
updated (though it depends maybe on how the new directories are generated).

Updated MKIOCCCENTRY_VERSION to "1.2.36 2025-02-28".
Updated TXZCHK_VERSION to "1.1.15 2025-02-28".
Updated CHKENTRY_VERSION to "1.1.6 2025-02-28".
Updated CHKENTRY_TEST_VERSION to "1.1.1 2025-02-28".