feat: Export THEME from the package (#4055)

* Use Theme type everywhere
* Rename Appearance type to Theme for consistency
* Reorder headers in readme
The host don't need to pass hardcoded strings any more and instead can use the exported constant

src/types.ts

@@ -10,6 +10,7 @@ import {
   Arrowhead,
   ChartType,
   FontFamilyValues,
+  Theme,
 } from "./element/types";
 import { SHAPES } from "./shapes";
 import { Point as RoughPoint } from "roughjs/bin/geometry";
@@ -98,7 +99,7 @@ export type AppState = {
   showHelpDialog: boolean;
   toastMessage: string | null;
   zenModeEnabled: boolean;
-  theme: "light" | "dark";
+  theme: Theme;
   gridSize: number | null;
   viewModeEnabled: boolean;
 
@@ -192,7 +193,7 @@ export interface ExcalidrawProps {
   zenModeEnabled?: boolean;
   gridModeEnabled?: boolean;
   libraryReturnUrl?: string;
-  theme?: "dark" | "light";
+  theme?: Theme;
   name?: string;
   renderCustomStats?: (
     elements: readonly NonDeletedExcalidrawElement[],