feat: add renderTopRight prop & remove GH corner from core (#3539)

* feat: add `renderTopRight` prop & remove GH corner from core

* reuse `--space-factor` var

* update readme & changelog

src/types.ts

@@ -182,6 +182,7 @@ export interface ExcalidrawProps {
     data: ClipboardData,
     event: ClipboardEvent | null,
   ) => Promise<boolean> | boolean;
+  renderTopRight?: (isMobile: boolean, appState: AppState) => JSX.Element;
   renderFooter?: (isMobile: boolean) => JSX.Element;
   langCode?: Language["code"];
   viewModeEnabled?: boolean;