`object_length_linter` should detect generics

As the `object_name_linter` already does, class suffixes for S3 generics shouldn't be part of the linted symbol.
That is, `object_length_linter(30)` (the default) shouldn't lint this example:

```
my_generic_function <- function(x, ...) {
  UseMethod("my_generic_function")
}

my_generic_function.my_cool_class <- function(x, ...) {
  cat("How cool and generic!\n")
}
```