Add system tools for interacting with directories

[tylerslaton]

There is currently no native way to interactive with directories in GPTScript.

If we add this, I'd like to see the following implemented as a first pass:

• sys.mkdir
• sys.ls
• sys.readdir

[njhale]

As of today, sys.ls exists, but sys.mkdir and sys.readdir do not.

[cjellick]

What would read.dir do?