Array, Dictionary: add into_read_only() + is_read_only()

[Bromeon]

Modeled Godot's make_read_only() as into_read_only(), so it can be used directly on construction:

let array = array![1, 2].into_read_only();

I don't expect people to use this that much in Rust, but it's a Godot feature that we should support, and "read-only-ness" can also have an impact on other functions.

As best-effort, this PR also adds panics in Debug mode if a mutating method is called on a read-only Array/Dictionary.

Should mark the last item on the original builtin feature list and thus closes #310.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1096