Exclude unnecessary DLLs from a build (#138)

* Fix compiler errors for Unity 2019.4

* prevent Json from IL2CPP code stripping (tested on 2019.4 IL2CPP, .NET Standard 2.0)

* Exclude NSubstitute dependencies (Tasks.Extensions, CompilerServices.Unsafe) from build + change NativeWebSocket to be only included for WebGL

Author: sierpinskid

…ibs/Websockets/NativeWebSocketWrapper.cs …ativeWebSocket/NativeWebSocketWrapper.csAssets/Plugins/StreamChat/Libs/Websockets/NativeWebSocketWrapper.cs renamed to Assets/Plugins/StreamChat/Libs/NativeWebSocket/NativeWebSocketWrapper.cs Assets/Plugins/StreamChat/Libs/Websockets/NativeWebSocketWrapper.cs renamed to Assets/Plugins/StreamChat/Libs/NativeWebSocket/NativeWebSocketWrapper.cs

@@ -1,3 +1,15 @@
-{
-	"name": "endel.nativewebsocket"
-}
+{
+    "name": "endel.nativewebsocket",
+    "references": [],
+    "includePlatforms": [
+        "WebGL"
+    ],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": false,
+    "defineConstraints": [],
+    "versionDefines": [],
+    "noEngineReferences": false
+}