Add "Convert List Bullets to Em Dash" command

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

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

src/formatter.ts

@@ -16,3 +16,9 @@ export function transformText(text: string, style: FormatStyle): string {
 export function cleanText(text: string): string {
     return [...text].map(ch => UNICODE_TO_ASCII_MAP.get(ch) ?? ch).join("");
 }
+
+export function bulletToEmdash(text: string): string {
+    return text.split("\n").map(line =>
+        line.startsWith("* ") ? "— " + line.slice(2) : line
+    ).join("\n");
+}