possible extension for class_equals_linter

[m-muecke]

I came across the following pattern for checking class membership:
is.element("msts", class(object$x)), which is equivalent to "msts" %in% class(object$x), but not yet checked by the class_equals_linter. Perhaps this could be an additonal check.

[MichaelChirico]

SGTM! Do you want to have a go at a PR?