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

Refactor `fluent_embed_derive` tests

Adds more tests and places shared logic behind common module

Created by  finchie  on May 6, 2025
MABGENI7CW5F5D3BFUJ7BS2H7XPYG4F3UMWGDPFFSMCCZKUUDLDQC
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