Add "Convert List Bullets to Arrow" command

Adds bulletToArrow() to formatter.ts and registers the new
unicode-formatter:bullet-to-arrow command. Updates README accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/formatter.ts

@@ -22,3 +22,9 @@ export function bulletToEmdash(text: string): string {
         line.startsWith("* ") ? "— " + line.slice(2) : line
     ).join("\n");
 }
+
+export function bulletToArrow(text: string): string {
+    return text.split("\n").map(line =>
+        line.startsWith("* ") ? "→ " + line.slice(2) : line
+    ).join("\n");
+}