feat: Translatable diagnostics

[Brayan-724]

This PR introduces infrastructure to make diagnostics in rust-analyzer translatable using Fluent.

Overview

• Diagnostic messages are now externalized and identified by Fluent message IDs.
• Adds basic integration with Fluent for message resolution.
• No translations are included yet — this just sets up the system.

Motivation

Enables future localization of diagnostics, improving accessibility for non-English users. This is not focused on built-in localization, but forks with their own messages.

Next Steps

• Add actual .ftl files for all the diagnostics.
• Expand Fluent usage to other user-facing strings.

[Brayan-724]

This is a proof of concept, so please give me feedback to continue

[blyxyas]

Feedback as an outsider to RA: If you're going to use fluent, it could be interesting seeing if rustc_fluent_macro could be added to auto-publish and have static checking support. Also, adding two whole modules just for fluent is probably not the most ergonomic solution.

[blyxyas]

Rustc errors do not change with translations

[blyxyas]

Some concerns I brought up in Discord, I saw the PR before reading that it's meant to be forked:

• If this is meant to be forked, how would the users get the forks? Would them download possibly unmaintained or with variable levels of quality? What if a maintainer stops maintaining their fork for whatever reason? The users would have to move to another fork.

• If an user wants to have multiple languages, they'd need to change binaries in their toolchain directory, and they would unsync again after an update, we'd need to talk with Rustup.

The safest bet for implementing multilanguage solutions is letting the community create language packs and linking rust-analyzer against them. This would impact performance but if it's on server startup, I don't think that the time for parsing and integrating the error messages is that important.

I'm not sure if Rust wants to have 5-10 forks with variable levels of maintability (without accounting for a possible vector of attack, because we'll have to warn that they're user maintained and watch them closely to avoid someone making a fork that mines bitcoin in user's machines)