Consider a code generation type safe version of i18n-embed-fl

The https://github.com/baptiste0928/rosetta project brings some interesting ideas, it generates code using the `build.rs` script that can be used to reference localizations using native Rust types which also integrates well with IDE autocomplete. In addition, the function signature for localizations with variables are also generated, making it very easy to use a given localization correctly without needing to reference the source file. Seems like it should be possible to do something similar with `fluent`.

Start by reviewing the linked rosetta project and its build.rs code-generation approach, then compare it with this project's Fluent localization handling. Define what generated Rust types and localization function signatures would be needed, and consider the generated API complete when Fluent messages can be referenced with native types and IDE autocomplete.

Written by the indexing model from the issue text.