Initial json server implementation

Completed the initial implementation of the JsonServer class, handling
the worker thread that runs the async task, as well as the connection
handler callback

The current handler awaits readline to cleanly exit when the connection
is closed, no data ingress is happening yet

Next steps include gating the start procedure to wait for everything to
start proprerly in order to avoid having start exit early and async
tasks crashing later

src/dynalab_core/errors.py

@@ -0,0 +1,7 @@
+# Copyright (C) 2026 Hector van der Aa <hector@h3cx.dev>
+# Copyright (C) 2026 Association Exergie <association.exergie@gmail.com>
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+
+class StateError(Exception):
+    """DynaLab Core state error."""