Add a native Luau (.d.luau) backend for LAD files

[jonasdedden]

Add a native Luau (.d.luau) backend for LAD files

Problem

ladfile_builder ships LAD post-processors for the Lua Language Server (--- @class .lua) and mdbook, but there's no native Luau one. luau-lsp can't consume the LuaLS dialect — it expects declare class … end / declare name: T syntax.

The luau runtime feature already exists, so you can run Luau scripts against Bevy, but there's currently no way to type-check them against the reflection registry.

Proposal

A native Luau backend in ladfile_builder, behind a luau_files feature, mirroring how lua_language_server_files is wired:

• a pure lad_to_luau(&LadFile, …) -> String conversion, and
• a LuauLadPlugin implementing LadFilePlugin, added to default_processors().

It would be a feature-gated module rather than a separate crate (like the LuaLS backend), since it's pure string generation over the ladfile types and needs no new dependencies.

Scope of a first cut:

• declare class for reflected types (with focus_crates to keep output readable), host global functions, and non-static instance handles like world;
• handle Luau grammar edges (reserved words, the unit type, static-vs-instance globals);
• unknown types resolve to any.