Use single PIO manifest

Fixes: https://github.com/me-no-dev/ESPAsyncWebServer/issues/223

library.json

@@ -0,0 +1,29 @@
+{
+  "name":"ESPAsyncWebServer",
+  "description":"Asynchronous HTTP and WebSocket Server Library for ESP8266 and ESP32",
+  "keywords":"http,async,websocket,webserver",
+  "authors":
+  {
+    "name": "Hristo Gochkov",
+    "maintainer": true
+  },
+  "repository":
+  {
+    "type": "git",
+    "url": "https://github.com/me-no-dev/ESPAsyncWebServer.git"
+  },
+  "version": "1.1.0",
+  "license": "LGPL-3.0",
+  "frameworks": "arduino",
+  "platforms":"espressif8266, espressif32",
+  "dependencies": [
+    {
+      "name": "ESPAsyncTCP",
+      "platforms": "espressif8266"
+    },
+    {
+      "name": "AsyncTCP",
+      "platforms": "espressif32"
+    }    
+  ]
+}