CmdBase / cmds.projects.Cmd: Add modules

Lots of sub- and sub-subcommands are derived from the base class of
the invoking command, notably below cmds.projects. That provides some
properties shared across the ancestor hierarchy of a command, but is
semantically unsound. Introduce jw.pkg.BaseCmd class as a place to
provide basic helpers shared across all commands used in a
jw.pkg.App's context. Also add cmds.projects.Cmd to be used by other
commands in cmds.projects in later commits.

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/CmdBase.py

@@ -0,0 +1,13 @@
+from .App import App
+from .lib.Cmd import Cmd as Base
+
+class CmdBase(Base):
+
+    @property
+    def app(self) -> App:
+        ret = super().app
+        if not isinstance(ret, App):
+            raise TypeError(
+                f'Expected {self.__class__.__name__}, got {type(ret).__name__}'
+            )
+        return ret