chore: add test for Keyword::lookup_keyword completeness

[TomAFrench]

Related issue(s)

Addresses review comment #903 (comment)

Description

Summary of changes

I've added a test so that the std::fmt:Display of any Keyword can be looked up to recover the original keyword token.

This follows the pattern used here: https://github.com/noir-lang/acvm/blob/f57ba57c2bb2597edf2b02fb1321c69cf11993ee/acir/src/circuit/black_box_functions.rs#L200-L211

Keyword::lookup_keyword also includes some non-keyword entries which might benefit from being split off. Tthey don't entirely gel with the purpose of this function and require us to return an Option<Token> rather than an Option<Keyword> which is what I would expect.

Dependency additions / changes

I've added strum as a dev dependency

Test additions / changes

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.
• This PR requires documentation updates when merged.

Additional context