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/excalidraw-app/components/GitHubCorner.tsx

@@ -0,0 +1,44 @@
+import oc from "open-color";
+import React from "react";
+
+// https://github.com/tholman/github-corners
+export const GitHubCorner = React.memo(
+  ({ theme, dir }: { theme: "light" | "dark"; dir: string }) => (
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      width="40"
+      height="40"
+      viewBox="0 0 250 250"
+      className="rtl-mirror"
+      style={{
+        marginTop: "calc(var(--space-factor) * -1)",
+        [dir === "rtl"
+          ? "marginLeft"
+          : "marginRight"]: "calc(var(--space-factor) * -1)",
+      }}
+    >
+      <a
+        href="https://github.com/excalidraw/excalidraw"
+        target="_blank"
+        rel="noopener noreferrer"
+        aria-label="GitHub repository"
+      >
+        <path
+          d="M0 0l115 115h15l12 27 108 108V0z"
+          fill={theme === "light" ? oc.gray[6] : oc.gray[7]}
+        />
+        <path
+          className="octo-arm"
+          d="M128 109c-15-9-9-19-9-19 3-7 2-11 2-11-1-7 3-2 3-2 4 5 2 11 2 11-3 10 5 15 9 16"
+          style={{ transformOrigin: "130px 106px" }}
+          fill={theme === "light" ? oc.white : "var(--default-bg-color)"}
+        />
+        <path
+          className="octo-body"
+          d="M115 115s4 2 5 0l14-14c3-2 6-3 8-3-8-11-15-24 2-41 5-5 10-7 16-7 1-2 3-7 12-11 0 0 5 3 7 16 4 2 8 5 12 9s7 8 9 12c14 3 17 7 17 7-4 8-9 11-11 11 0 6-2 11-7 16-16 16-30 10-41 2 0 3-1 7-5 11l-12 11c-1 1 1 5 1 5z"
+          fill={theme === "light" ? oc.white : "var(--default-bg-color)"}
+        />
+      </a>
+    </svg>
+  ),
+);