This new wrapper type can be constructed via macros which invoke the
`gettext_extract` proc macro to extract the string literals for PO file
generation.
The type checking enabled by this wrapper should prevent trying to obtain
translations for a string for which none exist.
Because some strings (e.g. for completions) are not defined in Rust, but rather
in fish scripts, the `LocalizableString` type can also be constructed from
non-literals, in which case no extraction happens.
In such cases, it is the programmer's responsibility to only construct the type
for strings which are available for localization.
This approach is a replacement for the `cargo-expand`-based extraction.
When building with the `FISH_GETTEXT_EXTRACTION_FILE` environment variable set,
the `gettext_extract` proc macro will write the messages marked for extraction
to a file in the directory specified by the variable.
Updates to the po files:
- This is the result of running the `update_translations.fish` script using the
new proc_macro extraction. It finds additional messages compared to the
`cargo-expand` based approach.
- Messages IDs corresponding to paths are removed. The do not have localizations
in any language and localizing paths would not make sense. I have not
investigated how they made it into the po files in the first place.
- Some messages are reordered due to `msguniq` sorting differing from `sort`.
Remove docs about installing `cargo-expand`
These are no longer needed due to the switch to our extraction macro.
