add devtools package

com.unity.uiwidgets.devtools/Editor/Unity.UIWidgets.DevTools.Editor.asmdef

+{
+    "name": "Unity.UIWidgets.DevTools.Editor",
+    "references": [],
+    "includePlatforms": [
+        "Editor"
+    ],
+    "excludePlatforms": []
+}

com.unity.uiwidgets.devtools/package.json

+{
+	"name": "com.unity.uiwidgets.devtools",
+	"displayName":"DevTools",
+	"version": "1.5.4-preview.1",
+	"unity": "2019.4",
+	"description": "DevTools is an indispensable tool for development in UIWidgets",
+	"dependencies": {
+	}
+}