Use LocalizableString for gettext

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.

.github/workflows/main.yml

@@ -32,11 +32,8 @@ jobs:
     - name: make fish_run_tests
       run: |
         make -C build VERBOSE=1 fish_run_tests
-    - uses: dtolnay/rust-toolchain@stable
     - name: translation updates
       run: |
-        # Required for our custom xgettext implementation.
-        cargo install --locked --version 1.0.106 cargo-expand
         # Generate PO files. This should not result it a change in the repo if all translations are
         # up to date.
         # Ensure that fish is available as an executable.