Add a plugin schema

plugin-schema.json

+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "JSON schema for a Grasscutter Plugin",
+  "type": "object",
+  "additionalProperties": true,
+  "definitions": {
+    "plugin-name": {
+      "type": "string",
+      "pattern": "^[A-Za-z\\d_.-]+$"
+    }
+  },
+  "required": [ "name", "description", "mainClass" ],
+  "properties": {
+    "name": {
+      "description": "The unique name of plugin.",
+      "$ref": "#/definitions/plugin-name"
+    },
+    "mainClass": {
+      "description": "The plugin's initial class file.",
+      "type": "string",
+      "pattern": "^(?!org\\.bukkit\\.)([a-zA-Z_$][a-zA-Z\\d_$]*\\.)*[a-zA-Z_$][a-zA-Z\\d_$]*$"
+    },
+    "version": {
+      "description": "A plugin revision identifier.",
+      "type": [ "string", "number" ]
+    },
+    "description": {
+      "description": "Human readable plugin summary.",
+      "type": "string"
+    },
+    "author": {
+      "description": "The plugin author.",
+      "type": "string"
+    },
+    "authors": {
+      "description": "The plugin contributors.",
+      "type": "array",
+      "items": {
+        "type": "string"
+      }
+    },
+    "website": {
+      "title": "Website",
+      "description": "The URL to the plugin's site",
+      "type": "string",
+      "format": "uri"
+    }
+  }
+}
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