croak on an attempt to run a directory as a script

How many times have I meant to run "perl -I lib myprog" but instead
run "perl lib myprog" only to exit 0 because that's what perl does
when you try to run a directory as a script (at least on unix)?  Many.

perl should croak when instructed to execute a directory.
[perl #61362] suggests it already does so on Win32.  Now it does it
everywhere.  Tests not yet written.
Add a test to detect error when attempting to syntax-check a directory.

For: RT #61362
perldelta for #61362, croak on `perl directory`
expect a different error for `perl dir` on Win32

We've known that this is how Win32 behaves, as it was documented in
the ticket for which this is a fix.  I don't think it's worth the
bother of ensuring we get EISDIR, as long as we don't just exit
silently!