Add viaversion and viabackwards as provides into fabric.mod.json

Allows mods to commonly check for either viaversion or viabackwards to be loaded

src/main/resources/fabric.mod.json

@@ -62,5 +62,9 @@
   },
   "custom": {
     "vfp:implVersion": "${implVersion}"
-  }
+  },
+  "provides": [
+    "viaversion",
+    "viabackwards"
+  ]
 }