Allow extension of vec_approx_equal behavior

[brookslogan]

Converted from comment in the "default" branch of vec_approx_equal0:

    # XXX No special handling for any other types/situations. Makes sense for
    # unclassed atomic things; custom classes (e.g., distributions) might want
    # recursion / specialization, though. vec_approx_equal0 should probably be
    # an S3 method; see also `vctrs::vec_proxy_{equal,compare}` though they
    # might not be sufficient (e.g., for keyed data frames such as `epi_df`s
    # that should have strict & nonstrict columns).