compose: Implement mention, for mention syntax

Author: chrisbobbe

lib/model/compose.dart

@@ -1,5 +1,6 @@
 import 'dart:math';
 
+import '../api/model/model.dart';
 import '../api/model/narrow.dart';
 import 'narrow.dart';
 import 'store.dart';
@@ -175,3 +176,7 @@ Uri narrowLink(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
 
   return store.account.realmUrl.replace(fragment: fragment.toString());
 }
+
+String mention(User user, {bool silent = false}) {
+  return '@${silent ? '_' : ''}**${user.fullName}|${user.userId}**';
+}

test/model/compose_test.dart

@@ -300,4 +300,10 @@ hello
     // TODO other Narrow subclasses as we add them:
     //   starred, mentioned; searches; arbitrary
   });
+
+  test('mention', () {
+    final user = eg.user(userId: 123, fullName: 'Full Name');
+    check(mention(user, silent: false)).equals('@**Full Name|123**');
+    check(mention(user, silent: true)).equals('@_**Full Name|123**');
+  });
 }