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/element/types.ts

@@ -1,10 +1,11 @@
 import { Point } from "../types";
-import { FONT_FAMILY } from "../constants";
+import { FONT_FAMILY, THEME } from "../constants";
 
 export type ChartType = "bar" | "line";
 export type FillStyle = "hachure" | "cross-hatch" | "solid";
 export type FontFamilyKeys = keyof typeof FONT_FAMILY;
 export type FontFamilyValues = typeof FONT_FAMILY[FontFamilyKeys];
+export type Theme = typeof THEME[keyof typeof THEME];
 export type FontString = string & { _brand: "fontString" };
 export type GroupId = string;
 export type PointerType = "mouse" | "pen" | "touch";