Fix issue 11365: Allow D scripts have arbitrary names

When -run is used, don't check for the filename to end with .d, allow
any file names. Normal compilation will still check for file names to
have standard extensions.

This seems to be a good compromise, people thinking allowing arbitrary
names always can be harmful shouldn't fear this test, as it only affect
"scripts", but scripts still can be flexible enough to use whatever name
they like (as it should be).

This might need a documentation update, and extra review is welcome as
I'm not sure there are hidden interactions I missed.