better UX for gomuks built-in commands

Fixes #157

lib/models/composer_draft.dart

@@ -6,6 +6,7 @@ import "package:nexus/models/content/image_pack.dart";
 import "package:nexus/models/content/message.dart";
 import "package:nexus/models/content/poll_start.dart";
 import "package:nexus/models/relation_type.dart";
+import "package:nexus/models/text_command.dart";
 
 part "composer_draft.freezed.dart";
 
@@ -14,6 +15,7 @@ class const ComposerDraft({
   // Formatted like ":id#name#", see the composer's tagTextFormatter
   final String text = "",
   final IList<Tag> tags = const .empty(),
+  final IList<TextCommand> textCommands = const .empty(),
   final int? relatedEvent,
   final RelationType relationType = .reply,
 

lib/models/text_command.dart

@@ -0,0 +1,48 @@
+import "package:collection/collection.dart";
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+
+enum TextCommandGroup { messageType, format }
+
+// Prefixes gomuks parses out of sent text, in the order it expects them
+enum TextCommand(final String description, {final TextCommandGroup? group}) {
+  me("Send as an emote", group: .messageType),
+  notice("Send as a notice", group: .messageType),
+  unencrypted("Send unencrypted, even if the room is encrypted"),
+  rawinputbody("Use the text as the body as-is, rather than from the HTML"),
+  plain("Send without any formatting", group: .format),
+  html("Send with only HTML, without markdown", group: .format),
+  htmlmd("Send with both HTML and markdown", group: .format),
+  rainbow("Send in rainbow colors, with markdown", group: .format);
+
+  String get prefix => "/$name";
+
+  bool completes(String text) =>
+      text.startsWith("/") && prefix.startsWith(text.toLowerCase());
+
+  // Replaces any command from the same group, as gomuks only uses one
+  IList<TextCommand> addTo(IList<TextCommand> commands) => .new(
+    commands
+        .where(
+          (other) => other != this && (group == null || other.group != group),
+        )
+        .toIList()
+        .add(this)
+        .sortedBy((command) => command.index),
+  );
+
+  static String prefixOf(IList<TextCommand> commands) =>
+      commands.map((command) => "${command.prefix} ").join();
+
+  static ({IList<TextCommand> commands, String text}) parse(
+    String text, [
+    IList<TextCommand> commands = const .empty(),
+  ]) => switch (values.firstWhereOrNull(
+    (command) => text.toLowerCase().startsWith("${command.prefix} "),
+  )) {
+    final command? => parse(
+      text.substring(command.prefix.length + 1),
+      command.addTo(commands),
+    ),
+    null => (commands: commands, text: text),
+  };
+}