Support multiple function clauses in anonymous functions

[doorgan]

This adds support for anymous functions with multiple clauses.

Before this change, this code would fail to compile:

argument fn
  "foo" -> "bar"
  _ -> "default"
end

Contributor checklist

• [x] Bug fixes include regression tests
• [ ] Chores
• [ ] Documentation changes
• [x] Features include unit/acceptance tests
• [ ] Refactoring
• [ ] Update dependencies

[zachdaniel]

🚀Thank you for your contribution🚀