editorui: add override for GMPL model files

Newbyte requested to merge Newbyte/gnome-builder:mod-as-in-gmpl into main

Otherwise Builder thinks it's an audio file that it can't play.

Builder still displays a 🎵 as icon for .mod files. It's not a huge deal for me, but I understand if you'd rather have that fixed before merging. However, I'm not sure how to resolve that.

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