feat: Multiplayer (#2)

Make this game multiplayer.

Setup:
- `common` for common classes used both by the server and the client
- `game` the actual client
- `server` the server

Reviewed-on: https://gitea.local.jzitnik.dev/jzitnik/game/pulls/2
Co-authored-by: jzitnik-dev <email@jzitnik.dev>
Co-committed-by: jzitnik-dev <email@jzitnik.dev>

common/.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>