os: add split_path()

[kbkpbot]

This will help user get the filename without ext.

split_path will split opath into (dir,filename,ext).
Examples:

dir,filename,ext := os.split_path('/usr/lib/test.so')
assert [dir,filename,ext] == ['/usr/lib','test','.so']

Bug fix:
os.base(), os.file_name(), os.dir() all support both windows/non-windows path at the same time.

[huly-for-github]

Connected to Huly®: V_0.6-21960

[spytheman]

Excellent work.