Add minimal translation functionality without dependency on React

[ewajm-shopify]

Description

• Addresses ([react-i18n] Extract base i18n logic to allow it to be used in non-react JS projects #2513)

Per the final comment in that issue

There's a gap here for a simpler version of i18n that is designed for usage in libraries

I wanted to try and take a step in this direction by adding a somewhat more robust version of polaris's I18n approach (preserving support for pluralization and ordinals), in a place where it could be more accessible to other projects.

SimpleI18n provides the following:

• supports plurals and ordinals
• allows replacements using {} syntax only
• translations must be passed in by user
• multiple translation libraries may be provided and are assumed to be in 'primary locale -> fallback locale' order

One of my goals was to minimize repeated code between SimpleI18n and the react-I18n I18n object, which is why I didn't try to bring over the formatting functionality.