add thread support

Fixes #129

lib/models/requests/get_threads.dart

@@ -0,0 +1,17 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+
+part "get_threads.freezed.dart";
+part "get_threads.g.dart";
+
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const GetThreadsRequest({
+  required final String roomId,
+  final String? since,
+  final int limit = 20,
+}) with _$GetThreadsRequest {
+  Map<String, Object?> toJson() => _$GetThreadsRequestToJson(this);
+
+  factory GetThreadsRequest.fromJson(Map<String, Object?> json) =>
+      _$GetThreadsRequestFromJson(json);
+}

lib/models/requests/send_event.dart

@@ -14,7 +14,7 @@ class const SendEventRequest({
   // Merged into the content by toJson, as gomuks doesn't take these
   @JsonKey(includeToJson: false) final String? relatesTo,
   @JsonKey(includeToJson: false) final String? relationType,
-  @JsonKey(includeToJson: false) final String? replyTo,
+  @JsonKey(includeToJson: false) final Relation? reply,
   @JsonKey(includeToJson: false) final Mentions? mentions,
   final bool synchronous = false,
   final bool disableEncryption = false,
@@ -27,14 +27,14 @@ class const SendEventRequest({
       "content": {
         ...content,
         "m.mentions": ?mentions?.toJson(),
-        if (relatesTo != null || replyTo != null)
+        if (relatesTo != null || reply != null)
           "m.relates_to": {
             ...((content["m.relates_to"] as Map<String, dynamic>?) ?? {}),
             if (relatesTo != null) ...{
               "event_id": relatesTo,
               "rel_type": relationType,
             },
-            if (replyTo != null) "m.in_reply_to": {"event_id": replyTo},
+            ...?reply?.toJson(),
           },
       },
     };

lib/models/requests/send_message.dart

@@ -38,11 +38,18 @@ class const Mentions({
 class const Relation({
   required final String eventId,
   required final RelationType relationType,
+  final String? threadRoot,
+  final bool isFallingBack = false,
 }) with _$Relation {
   Map<String, dynamic> toJson() {
     switch (relationType) {
       case RelationType.reply:
         return {
+          if (threadRoot != null) ...{
+            "rel_type": "m.thread",
+            "event_id": threadRoot,
+            "is_falling_back": isFallingBack,
+          },
           "m.in_reply_to": {"event_id": eventId},
         };