Tell regexp-util to generate regex compatible with u flag (#16816)

Co-authored-by: fisker Cheung <lionkay@gmail.com>

tests/format/markdown/splitCjkText/non-bmp/format.test.js

@@ -0,0 +1,19 @@
+// This preamble is unnecessary after #16805
+const preamble = "a 字 a 字 a 字";
+const code = `${preamble}\n${"葛\u{e0100}\n".repeat(3)}${"𠮷\n".repeat(2)}終\n`;
+const output = `${preamble}${"葛\u{e0100}".repeat(3)}${"𠮷".repeat(2)}終\n`;
+
+runFormatTest(
+  {
+    importMeta: import.meta,
+    snippets: [
+      {
+        code,
+        name: "Make sure non-BMP CJK characters are treated as CJK",
+        output,
+      },
+    ],
+  },
+  ["markdown"],
+  { proseWrap: "never" },
+);