Add RIU to default_en.txt

[PeterKraus]

Hello pint devs! I'd like to add the dimensionless "refractive index units" into pint, as they are often present in raw data files from instruments such as Agilent chromatographs etc. Please let me know if I need to add any tests. Cheers!

• [ ] Closes # (insert issue number)
• [ ] Executed pre-commit run --all-files with no errors
• [ ] The change is fully covered by automated unit tests
• [ ] Documented in docs/ as appropriate
• [x] Added an entry to the CHANGES file

[MichaelTiemannOSC]

Native support for things that make 🌈 -- love it!!

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #1816 will not alter performance

_{Comparing PeterKraus:riu (84027fd) with master (cb0ec94)}

Summary

✅ 437 untouched benchmarks