Consider dropping `@fluent/langneg` in favor of self-hosted algorithm

The matcher provided by `@fluent/langneg` supports multiple strategies, and that adds unneeded complexity (extra code) that we don't need as we only use the `"lookup"` strategy.

We should try to understand if porting to JS-land a simplified version of https://github.com/projectfluent/fluent-langneg-rs (which, afaik, is what the fluent version is based on) would be worth the effort or not.