Experimenting with more structured ways to handle command-line input/output in Rust

Unify `fluent_embed` macro API as `localize()`

Creates a single function to call for the upcoming macro shim library. This should centralise logic in the library crate rather than the proc_macro crate, which should make things substantially easier to develop and test.

Created by  finchie  on February 17, 2024
5FIVUZYFLOZ2CCH4GCOQQZFL3GDEB23VJ7J6YUXQDZQEAQDB76DQC
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