[feat] add `@skip` support for baml `class`

[Elijas]

https://docs.boundaryml.com/ref/attributes/skip

Use case

a baml class may have fields that are used for jinja conditionals, but should not be included in the prompt

[linear]

BAML-388 [feat] add `@skip` support for baml `class`

[Elijas]

Related 🐍 [feat] expose baml @description, @alias, @skip, and /// docstrings in the generated Pydantic classes #1829

[Elijas]

Discussion https://discord.com/channels/1119368998161752075/1372524222894571520/1377567240282574878