core/cachew: rely on ~/.cache for default cache path

- rely on appdirs for default cache path instead of hardcoded /var/tmp/cachew
  technically backwards incompatible, but no action needed
  you might want to clean /var/tmp/cachew after updating

- use default cache path (e.g. ~/.cache) by default
  see https://github.com/ActiveState/appdirs#some-example-output for more info
  *warning*: things will be cached by default now (used to be uncached before)

- treat cache_dir = None in the config
  *warning*: kind of backwards incompatible.. but again nothing disasterous

my/core/common.py

@@ -214,7 +214,8 @@ if TYPE_CHECKING:
 
     mcachew: McachewType
 
-# TODO set default cache dir here instead?
+
+# TODO I don't really like 'mcachew', just 'cache' would be better... maybe?
 # todo ugh. I think it needs doublewrap, otherwise @mcachew without args doesn't work
 def mcachew(*args, **kwargs): # type: ignore[no-redef]
     """