[BUGFIX] Add missing `Icon::SIZE_MEGA` constant

The Icon API in general knows the icon size "mega", which renders an
icon in the dimensions 64x64. However, the according constant was absent
in the `Icon` class, which is now added.

Resolves: #101431
Releases: main, 12.4
Change-Id: Ib889a1f8862591a4c78453ac24e15ce869781715
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80135


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>