Rust 1.80 stabilized `std::sync::LazyLock`, which replaces
`once_cell::sync::Lazy`. There is one exception in
`src/env_dispatch.rs`, which still uses the `once_cell` variant, since
the code there relies on `Lazy::get`, which also exists for `LazyLock`,
but will only be stabilized in Rust 1.94, so we can't use it yet.
Part of #12289
Extract the language selection code from the gettext crate, and to a
lesser extent from `src/localization/mod.rs` and put it into
`src/localization/settings.rs`. No functional changes are intended.
Aside from better separation of concerns, this refactoring makes it
feasible to reuse the language selection logic for Fluent later on.
Part of #12190
This is part of the larger effort of splitting up fish's huge main crate
to improve incremental build speed.
We could extract more logic from `src/wutil/gettext.rs` into the new
crate, but this would require putting wide-string handling into that
crate, which I'm not sure we want. Doing so would have the advantage
that crates which don't depend on fish's main crate (i.e. all crates
other than fish's main crate itself and the binary crates built on top
of it) could then localize messages as well. This will be less relevant
if we replace gettext with Fluent for messages originating from the Rust
sources.
Closes#12108
