Add GodotImmutable to constrain #[opt(default)] types

[Bromeon]

Addresses some of the open questions from #1396 regarding mutable default values.
Conservatively disallows many types, we can probably open them up over time.

One future approach would be to allow immutable values where types support them, e.g.:

#[opt] object: Option<Gd<RefCounted>>,
// We can safely use None here.

#[opt] v: Variant,
// We can safely use Variant::nil() here. Recreated each time.

Or, have an override like:

#[opt(default = allow_mut(...))] object: Gd<RefCounted>,

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1406

[Yarwin]

bool?