Remove unnecessary unit struct fields from `Units` enum variants

[Turbo87]

The unit struct fields in Units enum variants were never accessed - they were only used for construction and immediately discarded in pattern matching.

Changes:

• Convert tuple-like enum variants (e.g., steradian(steradian)) to unit-like variants (e.g., steradian)
• Update pattern matching to use simpler syntax without wildcards
• Update ALL_UNITS initialization to use unit-like syntax
• Update tests to construct enum variants without passing unit structs

Benefits:

• Simpler, clearer API
• Smaller enum variants (no tuple wrapper)
• Less confusion about unused fields

Breaking change: External code constructing Units enum variants must use unit-like syntax instead of tuple-like syntax.

Fixes #539