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

Rename `fluent_embed_runtime` to `fluent_embed`

The previous fluent_embed (which contains the proc-macro code) has become fluent_embed_derive. Since there are runtime dependencies, the user will be expected to add fluent_embed and use the re-exported macro rather than depending on it directly.

Created by  finchie  on July 29, 2024
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