Add cargo feature for enabling gettext extraction

This allows having the proc macro crate as an optional dependency and speeds up
compilation in situations where `FISH_GETTEXT_EXTRACTION_FILE` changes, such as
the `build_tools/check.sh` script. Because we don't need to recompile on changes
to the environment variable when the feature is disabled, cargo can reuse
earlier compilation results instead of recompiling everything.
This speeds up the compilation work in `build_tools/check.sh` when no changes
were made which necessitate recompilation.
For such runs of `build_tools/check.sh`, these changes reduce the runtime on my
system by about 10 seconds, from 70 to 60, approximately.
The difference comes from the following two commands recompiling code without
the changes in this commit, but not with them:
- `cargo test --doc --workspace`
- `cargo doc --workspace`

build_tools/update_translations.fish

@@ -29,7 +29,7 @@
 # `FILE` must be the path to a gettext template file generated from our compilation process.
 # It can be obtained by running:
 #   set -l FILE (mktemp)
-#   FISH_GETTEXT_EXTRACTION_FILE=$FILE cargo check
+#   FISH_GETTEXT_EXTRACTION_FILE=$FILE cargo check --features=gettext-extract
 
 # The sort utility is locale-sensitive.
 # Ensure that sorting output is consistent by setting LC_ALL here.