build: Generate single executable for use #12

For people using the language but not developing it, there should be an easy way to download and use the interpreter and library.

On Linux this probably means bundling up the Python code and using Python as the interpreter.

This manages to create a 'NewLang' executable containing the interpreter:

rm -r src/__pycache__
python3 -m zipapp src -p "/usr/bin/env python3" -c -m "main:main" -o NewLang

This should also support bundling in NewLang code too.

Okay, 'make build' will now create a 'NewLang' executable.