main.py: Support running using a shebang

code.txt: Replace 'Done' with 'EndIf' and 'EndSet' ...

This is much less ambiguous to humans the parser.

code.txt: Switch from 'Is' syntax to 'Set To' ...

'Subject Is Command' is hard to distinguish from 'Subject Verb Arguments'.
Let's do 'Set Subject To Command Done' instead.

Initial WIP commit