feat: add shell commands

Also switch to classes

src/subcommands/screenshot.py

@@ -1,5 +1,11 @@
 from argparse import Namespace
 
 
-def run(args: Namespace) -> None:
-    pass
+class Command:
+    args: Namespace
+
+    def __init__(self, args: Namespace) -> None:
+        self.args = args
+
+    def run(self) -> None:
+        pass