Allow individual files to be added to the shell path

Right now, the shell.resolveProgram function will check each path entry as if it's a directory via fs.combine. My suggestion is that shell.resolveProgram will also check whether the file name specified by the entry (via fs.getName) is the same as the argument, so individual files can be added to the path.

For example, adding /folder/myProgram to the path and then running shell.resolveProgram("myProgram") would return /folder/myProgram.

With this change, you would not have to add entire directories to the path when but one program is needed from that folder.