Common.File: Replaced a wrong (?) implementation of extname with one from System.FilePath

I tested the function with QuickCheck and got a deviation:

> quickCheck (\s-> FilePath.takeExtension s == extname s)
> *** Failed! Falsifiable (after 96 tests and 10 shrinks):
> "./"

Nevertheless, the koka compiler seems to work the FilePath.takeExtension function.
I'll therefore do the replace and will fix any resulting issues when they occur.