Heads-up: new virtual category, "education"

[Pau Amma]

As discussed previously in the thread starting in 
https://lists.freebsd.org/pipermail/freebsd-ports/2020-July/118989.html, 
the ports initially in that category will be astro/marble, 
astro/nightfall, biology/figtree, biology/py-scikit-bio, cad/basicdsp, 
cad/feappv, cad/logisim, comms/gtkmmorse, deskutils/docear, 
deskutils/fet, devel/elfkickers, devel/raylib, devel/thonny, 
emulators/edumips64, games/anki, games/colobot, games/crashtest, 
games/crrcsim, games/gcompris-qt, games/kanagram, games/katomic, 
games/nimuh, games/py-mnemosyne, games/robocode, games/robocode-naval, 
games/tuxmath, games/tuxtype, graphics/geomview, graphics/libreatlas, 
japanese/kiten, japanese/slimeforest, lang/kturtle, 
math/R-cran-LearnBayes, math/analitza, math/geogebra, math/geonext, 
math/kalgebra, math/kbruch, math/kig, math/kmplot, 
math/octave-forge-ltfat, math/rocs, math/timbl, math/ump, 
misc/artikulate, misc/kdeedu, misc/kgeography, misc/klettres, 
misc/ktouch, misc/kwordquiz, misc/lingoteach, misc/parley, misc/paukern 
misc/py-orange3-educational, multimedia/projectx, net/ns3, print/jabref, 
science/fisicalab, science/kalzium, science/libccp4, science/py-qspin, 
science/step, sysutils/hourglass, textproc/popup, textproc/py-nltk, 
www/efront, www/ilias, www/ilias6, www/moodle35, www/moodle37, 
www/moodle38, www/moodle39, and www/sakai.

Bugs filed: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248402 
(for the ports tree) and 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=248403 (for the 
Porter's Handbook).