feat!: force users to explicitly enable code execution

[mfontanini]

This changes the code snippet execution so that it is disabled by default and users need to opt in by either:

• Running presenterm with the -x flag.
• Set the snippet.exec.enable property to true in their config files.

This aims to make running untrusted presentations more secure. You can always set the property in your config file if you only ever run your own presentations, but by default this prevents opening a presentation someone else wrote and accidentally executing code in one of the slides that does something malicious.