feat: BREAKING improve command_extension

- unify behavior of all message sending related command
- add a StringBuffer as stdout-like output buffer for commands
- create a typedef for the command function signature
- create a common exception type for command execution
- enable commands to run on Client-level rather than Room-level
- BREAKING: Client.addCommand signature now takes an optional StringBuffer as second parameter

lib/src/client.dart

@@ -98,7 +98,7 @@ class Client extends MatrixApi {
   DateTime? _accessTokenExpiresAt;
 
   // For CommandsClientExtension
-  final Map<String, FutureOr<String?> Function(CommandArgs)> commands = {};
+  final Map<String, CommandExecutionCallback> commands = {};
   final Filter syncFilter;
 
   final NativeImplementations nativeImplementations;

lib/src/room.dart

@@ -625,6 +625,7 @@ class Room {
     String msgtype = MessageTypes.Text,
     String? threadRootEventId,
     String? threadLastEventId,
+    StringBuffer? commandStdout,
   }) {
     if (parseCommands) {
       return client.parseAndRunCommand(
@@ -635,6 +636,7 @@ class Room {
         txid: txid,
         threadRootEventId: threadRootEventId,
         threadLastEventId: threadLastEventId,
+        stdout: commandStdout,
       );
     }
     final event = <String, dynamic>{