Is your feature request related to a problem? Please describe.
There is no Optifine format emmisive texture mods for Fabric. The closest are Continuity (which adds Optifine connected textures) and CTM Fabric (which implements a different format). When I requested the devs of Continuity implemented emmisive textures, they said they were unable to due to API limitations. With how Iris already works, I think that it can bypass the limitations that the devs mentioned.

Describe the solution you'd like
I would like Iris to add support for Optifine Emmisive textures.

Describe alternatives you've considered
Using CTM Fabric texture packs.

Iris can not get around these limitations, it would still need to use the same APIs to do what Continuity wants to wait to do properly.