add rule definition with minimal dsl

lib/utils.ts

@@ -12,3 +12,11 @@ export function next<T>(iterator: Iterator<T>) {
     if (result === undefined) return null;
     else return result as T;
 }
+
+export function split_once(text: string, separator: string) {
+    const cursor = text.indexOf(separator);
+    if (cursor === -1) return [text, null] as const;
+    const left = text.slice(0, cursor);
+    const right = text.slice(cursor + separator.length);
+    return [left, right] as const;
+}