Multi Point Lines (based on Multi Point Arrows) (#660)

* Enable multi points in lines

* Stop retrieving arrow points for lines

* Migrate lines to new spec during load

* Clean up and refactor some code

- Normalize shape dimensions during load
- Rename getArrowAbsoluteBounds

* Fix linter issues

src/element/sizeHelpers.ts

@@ -1,6 +1,9 @@
 import { ExcalidrawElement } from "./types";
 
 export function isInvisiblySmallElement(element: ExcalidrawElement): boolean {
+  if (element.type === "arrow" || element.type === "line") {
+    return element.points.length === 0;
+  }
   return element.width === 0 && element.height === 0;
 }