This patch updates the Font Awesome library version from 4.7 to 6.3.0.
The readme_moodle.txt has been updated properly because the new version
works slightly different.
There is a file to add backward compatibily to version 6 files (shim).
More information about how to upgrade from 4 to 6 can be found in
https://fontawesome.com/docs/web/setup/upgrade/upgrade-from-v4
Although a few icons have been renamed in Moodle 6, as the new version
is compatible with v4 icons, for now the references to the old icons
(fa fa-xxxx --> fa-solid fa-xxxx) haven't been changed yet.
A separate issue will be created to update them in the future and
review if some new icons can be used too.
We add a new theme config so the theme can say it supports font-awesome.
If this is true, the pix_icon renderer will call a mapping function to map
from the moodle style t/edit to a font-awesome style fa-cog icon name. Then the renderer
will either render an image tag for old icons - or an accessible font-awesome <i> tag.
This mostly works - but there are some places where we don't use the pix icon renderer, and
we directly create image tags with pix_url image sources. These will need updating (Atto icons,
drag and drop move icons, editpdf icons).
