Port TraitItem::TraitFn to be stored in the parsed declaration engine.

[tritao]

Description

This PR ports TraitItem::TraitFn to be stored in the parsed declaration engine, thus allowing it to be referenced as a parsed declaration id, which will be required by a future AST ordering pass.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.