Support Unicode micro prefix symbols

[Mirochill]

Summary

• register the common Unicode micro symbol variants as micro prefix aliases
• add prefixed abbreviation aliases such as µm/μm to the same unit entry as um
• let the unit parser consume Unicode unit tokens instead of limiting tokens to ASCII letters
• add regression coverage for U+00B5, U+03BC, and mathematical mu variants

Closes #13

Validation

• git diff --check HEAD~1..HEAD
• reviewed the parser and prefix registration paths with rg
• Tests not run locally.