Add opt-out for --no-* boolean switch aliases

[t-kalinowski]

Summary

Adds a negative_alias: false annotation for boolean switches so apps can expose only the positive flag form, such as --version, without also accepting or documenting --no-version.

Closes #24.

User-facing changes

• Boolean switches can now opt out of generated --no-* aliases with #| negative_alias: false.
• Help output omits the generated negative alias when disabled.
• Disabled negative aliases are rejected as unknown arguments.
• README and NEWS document the new annotation.

Internal changes

• Added shared has_negative_alias() logic using exact [[ lookup to avoid R $ partial matching.
• Updated argument parsing and help formatting to respect the annotation.
• Added regression coverage for disabled aliases, true-default switches, and accidental negative partial matching.

[t-kalinowski]

Closed in favor of #28