Validate class registrations against ClassDB

[Bromeon]

Godot's classdb_register_extension_class* functions return void and only print to stderr on failure, so a rejected class, method, property, signal or constant is silently missing at runtime. The printed Godot error may vary in detail.

New registry::reg_validation module queries ClassDB before each registration and reports a precise error via godot_error! (and in a potential future builder API would allow programmatic check).

In addition, it exits Godot after collecting all errors. Validation runs only under strict safeguards (default in Debug).

#[var] and #[func] are allowed to collide in Godot, but there aren't many use cases, and an explicit #[var(override)] or so might be better. For now warn (no breaking change), will become hard error in v0.6.

Here's a small manual test.

//! Deliberately broken registrations, to see diagnostics of `godot_core::registry::reg_validation`.
//!
//! Diagnostics only go to stderr (not tested). Enable the `mod` in the parent module, run `check.sh itest` and check stderr.
//! Both classes derive `Node`, since `ClassDB` is not reachable at `Core` init level.

use godot::prelude::*;

/// Error: class name already exists. Separate class, as the failed registration aborts before its symbols.
#[derive(GodotClass)]
#[class(init, base = Node, rename = Camera2D)]
struct DemoClassClash {}

/// Errors: duplicate method, property, signal.
/// Warnings: `duplicate`, `owner` and its accessors shadow `Node`.
#[derive(GodotClass)]
#[class(init, base = Node)]
struct DemoSymbolClashes {
    base: Base<Node>,

    #[var]
    same_prop: i32,

    // Explicit accessors: `rename` also renames generated ones, which would already clash at Rust level.
    #[var(get = get_other, set = set_other, rename = same_prop)]
    other_prop: i32,

    #[var]
    owner: i32,
}

#[godot_api]
impl DemoSymbolClashes {
    #[func]
    fn same_name(&self) {}

    #[func(rename = same_name)]
    fn other_name(&self) {}

    #[func]
    fn get_other(&self) -> i32 {
        self.other_prop
    }

    #[func]
    fn set_other(&mut self, value: i32) {
        self.other_prop = value;
    }

    #[func]
    fn duplicate(&self) {}

    #[signal]
    fn ready();
}

Closes #1024.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1664