CLN: add property to check if NDFrame uses ArrayManager
xref https://github.com/pandas-dev/pandas/pull/40482/files#r596222938

cc @jbrockmendel 
@jbrockmendel the property makes it more difficult for type checking, so not sure it's worth the change then for the few occurences > the property makes it more difficult for type checking

ok, can revisit if the number of occurrences increases