Improved Fullbright

This is an experimental pack utilizing the new `fullbright.fsh` shader added in 24w33a.

This shader allows the lightmap to be changed in real time using code from a resource pack, and is compatible with all mods, even Sodium.

This simple pack allows you to enable Fullbright by setting your brightness to 100%.

(The license is set to ARR becaue I wasn't sure what to use; it's pretty much just a few line changes on top of Vanilla's shader.)