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/constants.ts

@@ -70,6 +70,11 @@ export const FONT_FAMILY = {
   Cascadia: 3,
 };
 
+export const THEME = {
+  LIGHT: "light",
+  DARK: "dark",
+};
+
 export const WINDOWS_EMOJI_FALLBACK_FONT = "Segoe UI Emoji";
 
 export const DEFAULT_FONT_SIZE = 20;