Add support for negatable flags (--[no-]color)

[DannyBen]

The main motivation behind this feature, is the recently added argfile support, which means that users can define something like --color in their argfile, and then they have no way to disable it.

This can now be mitigated easily with:

flags:
- long: --color
  short: -c
  help: Enable color output
  negatable: true

---

• Implementation
• Specs
• Bashly bash completions
• Bashly CLI docs (bashly doc flag.negatable)
• JSON schema
• bashly.dev docs - Source | Preview