feat(plugins): add Velocity proxy and Fabric/Forge/NeoForge/Paper integration mods

Server-side integration plugins: the Velocity proxy plugin plus Fabric, Forge, NeoForge, and Paper mods with a shared module. Gradle build output is not tracked.

plugins/fabric/src/main/resources/fabric.mod.json

@@ -0,0 +1,19 @@
+{
+  "schemaVersion": 1,
+  "id": "felis-link",
+  "version": "0.1.0",
+  "name": "Felis Link",
+  "description": "In-game /link command: mints a one-time account-link code from felis-api.",
+  "authors": ["Felis"],
+  "license": "MIT",
+  "environment": "server",
+  "entrypoints": {
+    "server": ["best.lolicon.felis.fabric.FelisFabricMod"]
+  },
+  "depends": {
+    "fabricloader": ">=0.15.0",
+    "minecraft": "~1.20.1",
+    "java": ">=17",
+    "fabric-api": "*"
+  }
+}