api [nfc]: Rename RealmEmojiItem.id to emojiCode

This way the binding encapsulates an important fact about this
bit of the API -- namely that these values are the same as the
"emoji code" values seen elsewhere in the API -- so that we don't
have to re-verify it when working elsewhere in the codebase and
using this class.

Also edit a doc comment in the model code to take advantage of
that understanding.

lib/api/model/model.dart

@@ -115,15 +115,16 @@ class CustomProfileFieldExternalAccountData {
 /// in <https://zulip.com/api/register-queue>.
 @JsonSerializable(fieldRename: FieldRename.snake)
 class RealmEmojiItem {
-  final String id;
+  @JsonKey(name: 'id')
+  final String emojiCode;
   final String name;
   final String sourceUrl;
   final String? stillUrl;
   final bool deactivated;
   final int? authorId;
 
   RealmEmojiItem({
-    required this.id,
+    required this.emojiCode,
     required this.name,
     required this.sourceUrl,
     required this.stillUrl,
@@ -137,7 +138,6 @@ class RealmEmojiItem {
   Map<String, dynamic> toJson() => _$RealmEmojiItemToJson(this);
 }
 
-
 /// The name of a user setting that has a property in [UserSettings].
 ///
 /// In Zulip event-handling code (for [UserSettingsUpdateEvent]),

lib/model/emoji.dart

@@ -131,7 +131,7 @@ class EmojiStoreImpl with EmojiStore {
   /// The same as [PerAccountStore.realmUrl].
   final Uri realmUrl;
 
-  /// The realm's custom emoji, indexed by [Reaction.emojiCode],
+  /// The realm's custom emoji, indexed by their [RealmEmojiItem.emojiCode],
   /// including deactivated emoji not available for new uses.
   ///
   /// These are the emoji that can have [ReactionType.realmEmoji].

test/example_data.dart

@@ -77,7 +77,7 @@ RealmEmojiItem realmEmojiItem({
 }) {
   assert(RegExp(r'^[1-9][0-9]*$').hasMatch(emojiCode));
   return RealmEmojiItem(
-    id: emojiCode,
+    emojiCode: emojiCode,
     name: emojiName,
     sourceUrl: sourceUrl ?? '/emoji/$emojiCode.png',
     stillUrl: stillUrl,