mf.as_is_callable

When defining default variables during register(), i.e. using register_helpers, it is possible to specify if callables are parsed to resolve variable chains using register_helpers({...}, parsefn=False).

This MR adds a small QoL-Alias to include the same functionality into the variable definition itself, i.e.
instead splitting into two calls:

mf.register_helpers({
   "var": lambda x: 42
})
mf.register_helpers({
   "instance": MyClass()
}, parsefn=False)

this feature allows to merge such calls into a single one:

mf.register_helpers({
   "var": lambda x: 42
   "instance": mf.as_is_callable(MyClass())
})