Refactor ExcalidrawElement (#874)

* Get rid of isSelected, canvas, canvasZoom, canvasOffsetX and canvasOffsetY on ExcalidrawElement.

* Fix most unit tests. Fix cmd a. Fix alt drag

* Focus on paste

* shift select should include previously selected items

* Fix last test

* Move this.shape out of ExcalidrawElement and into a WeakMap

package.json

@@ -102,5 +102,8 @@
   "repository": {
     "type": "git",
     "url": "https://github.com/excalidraw/excalidraw.git"
+  },
+  "engines": {
+    "node": ">=12.0.0"
   }
 }