Append sys.append builtin

[njhale]

Add a new built-in tool called sys.append that appends content to a file instead of truncating it.

Resolves https://github.com/gptscript-ai/gptscript/issues/24

I think this could also be implemented in sys.write by adding an extra parameter to indicate that an append is desired. Interested to hear if folks think that's a better approach.