refactor: Use ESM (#8358)

* wip

* wip

* fix

* clean up

* Update tsconfig.json

* Update activitypub.ts

* wip

packages/backend/src/misc/acct.ts

@@ -0,0 +1,14 @@
+export type Acct = {
+	username: string;
+	host: string | null;
+};
+
+export function parse(acct: string): Acct {
+	if (acct.startsWith('@')) acct = acct.substr(1);
+	const split = acct.split('@', 2);
+	return { username: split[0], host: split[1] || null };
+}
+
+export function toString(acct: Acct): string {
+	return acct.host == null ? acct.username : `${acct.username}@${acct.host}`;
+}