Exclude all *.existing.json files from jar

[JT122406]

Kinda a extension of excluding .cache files from the jar but excludes the .existing.json files that some use for lang datagen from the jar as they cannot be used by the game

[modmuss50]

that some use for lang datagen

Is this something that Minecraft generates? Or is it just something that people use in their own mods. If the latter I dont think it should be in loom, its trivial to add to your own mod if needed. I also think its possible that people will have json files named like this that people will want to keep.