add replaceAsync util function

src/utils/replaceAsync.ts

@@ -0,0 +1,38 @@
+// asynchronous String#replace
+
+export default async function replaceAsync(
+	str: string,
+	re: RegExp,
+	func: (...any) => Promise<string>
+) {
+	const replacements: Promise<Replacement>[] = [];
+	str.replace(re, (...args) => {
+		replacements.push(
+			func(...args).then(
+				res =>
+					<Replacement>{
+						offset: args[args.length - 2],
+						length: args[0].length,
+						replacement: res,
+					}
+			)
+		);
+		return '';
+	});
+	let out = '';
+	let lastEnd = 0;
+	for (const { offset, length, replacement } of await Promise.all(
+		replacements
+	)) {
+		out += str.slice(lastEnd, offset) + replacement;
+		lastEnd = offset + length;
+	}
+	out += str.slice(lastEnd);
+	return out;
+}
+
+interface Replacement {
+	offset: number;
+	length: number;
+	replacement: string;
+}