Move scene, elements, and history into their respective modules

- Add "elements" argument to all scene functions to break reliance on global vars
- Move all scene functions under scene directory encapsulated by `index.ts`
- Move all element functions under `element` directory encapsulated by index.ts
- Create a SceneHistory class to store scene history state
- Add `createScene` function to create scene object

src/element/typeChecks.ts

@@ -0,0 +1,7 @@
+import { ExcalidrawElement, ExcalidrawTextElement } from "./types";
+
+export function isTextElement(
+  element: ExcalidrawElement
+): element is ExcalidrawTextElement {
+  return element.type === "text";
+}