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

Simplify `fluent_embed::group` module

Replace borrowed strings (&str) with owned Strings, and remove the MessageEntry wrapper in favour of fluent_syntax::ast::Message. These may be re-visited in the future but for now are mostly unnecessary complexity that is blocking a file-oriented API, which is how the library will actually be used.

Created by  finchie  on February 14, 2024
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