feat!: require fallbacks in ` desc_localizations` implementations for localized commands

[circuitsacul]

• [x] DescriptionLocalizations and NameLocalizations structs
• [ ] Documentation for new structs?
• [x] Make desc and desc_localizations mutually exclusive
• [x] Write tests

[circuitsacul]

Can you update the unreleased section of the CHANGELOG.md file to include your changes?

I made a best-guess for how to format it. LMK if I should change it.

[baptiste0928]

It's great, thanks again for the PR :+1: