Add `enforceNonce` field to `MessageBuilder`

[MCausc78]

Description

Implements https://github.com/discord/discord-api-docs/pull/6647

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] Ran dart analyze or make analyze and fixed all issues
• [x] Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works (TODO, see https://github.com/discord/discord-api-docs/pull/6647#issuecomment-1934896444)