Replace rust-i18n with i18n-embed

[nc7s]

As discussed in #1416. From there:

I'm also tempted to just do something akin to your patch, as doing this translation is simple enough that pulling in dependencies may be overkill.

Probably, if strings are to be fairly simple and keep that way. Optimize with phf, even.

Anyway, the new code is here for comparison.

This is a very crude draft with quite some points up for debate: code style and placement, workaround for locale override with config value (there's a comment about this), structure of i18n resources, and MSRV (enabling std::sync::LazyLock, but actually not very relevant to i18n).

Good news is this does not significantly affect compiled size (even down a few kilos actually).

[fujiapple852]

Thanks, I'll review this properly later today.

One thing worth doing is re-introducing the locale::t! "wrapper" macro such that no changes are needed at the call site (i.e. everything in frontend::render).

I tested this locally and it seemed to work ok:

#[macro_export]
macro_rules! t {
    ($($all:tt)*) => {
        i18n_embed_fl::fl!($crate::locale::LANGUAGE_LOADER, $($all)*)
    }
}