feat: convert serial console into the serial plugin

plugins/serial/CHANGELOG.md

@@ -0,0 +1,8 @@
+# Changelog
+
+## 1.0.0
+
+Converted from core's `src/backend/hosts/serial.ts` and
+`src/ui/features/serial/` in Termix 2.9.0. Its own port (30011) and nginx
+blocks are gone; it now serves `/plugin-ws/serial/console` like every other
+plugin.