Support multierr.Every about multierr

[tchung1118]

We agree that this would be a good convenience feature to add to multierr, but we're still considering if this should work with errors.Is or should be provided as a separate function from this package.

[abhinav]

FWIW, the standard library's errors.Join matches multierr's behavior.
errors.Is will return true if any of the errors in a errors.Join-ed error matches.
Despite the name, errors.Is is not an equality check but an approximate match.

Perhaps we need an "all" variant of Is (multierr.AllAre or multierr.Every?), where every error in the multierr list gets an errors.Is call individually, but not at the top-level of the multierr.

[r-hang]

Internal ref: GO-1932