improve & granularize ExcalidrawElement types (#991)

* improve & granularize ExcalidrawElement types

* fix incorrectly passing type

* fix tests

* fix more tests

* fix unnecessary spreads & refactor

* add comments

src/global.d.ts

@@ -5,3 +5,7 @@ interface Window {
 interface Clipboard extends EventTarget {
   write(data: any[]): Promise<void>;
 }
+
+type Mutable<T> = {
+  -readonly [P in keyof T]: T[P];
+};