[file icons] Adding default fileicon support to language contributions

It would be nice if extensions could specify custom file icons for the
file types it supports. For visual consistency this could be implemented
to only be used as a fallback if the user's selected theme does not specify
an icon.

[paniq]

yep. been very sad today to realize i couldn't add a file icon for my custom language. freedesktop icon themes has a very sensible implementation of all this, so i expected it to work here in a similar way as well, but no. sad sad sad.

[AlexShemeshWix]

+1