Migrate domWidget from js to ts (#3)

* Rename js to ts

* Migrate domWidget.js

* Fix global declare

tsconfig.json

@@ -20,7 +20,8 @@
     "baseUrl": ".",
     "paths": {
       "*": ["src/*"],
-    }
+    },
+    "typeRoots": ["src/types", "node_modules/@types"]
   },
-  "include": ["src/**/*"]
+  "include": ["src/**/*", "src/types/**/*.d.ts"],
 }