Support @export_node_path attribute

[tuto193]

Added support for export_node_path annotation (Completes #1128)

Adds the ability to annotate fields with @export_node_path:

#[derive(GodotClass)]
#[class(base=Node2D)]
struct SomeClass {
    /// Equivalent to:
    /// ```gdscript
    /// @export_node_path("Button", "TouchScreenButton") var some_button: NodePath
    /// ```
    #[export(node_path = ("Button", "TouchScreenButton"))]
    some_button: NodePath,

    base: Base<Node2D>
}

Note: I didn't find the exact PR where export_node_path was introduced to GDScript upstream, so if it's needed let me know, and I will dig deeper. Otherwise let me know if I should adjust any of these changes.

Couldn't have done it without other PRs like those of @Swivelgames on #1183. It really made it a lot easier to have that as reference, so thanks to them a lot!

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1620

[Bromeon]

Thanks a lot, this is a nice addition! 👍

PR looks mostly good, added some comments on smaller parts.

[Bromeon]

Is this hand-parsing of tokens really needed before you pass it to ListParser?

How do other godot-rust proc-macro APIs that deal with lists do it?

[tuto193]

I've been breaking my head a bit with this today. I'm not using the handle_list(), since I thought, we would allow
[Option 1]

  // ...
  #[export(node_path = "Class1")] // (not a list)
  // ...

as a valid case.

Otherwise I get the error:

...
error: expected list of items
    --> /Users/carlosfernandez/Git/gdext/target/debug/build/itest-c56514b7f101be8d/out/gen_ffi.rs:2412:30
     |
2412 |         #[export(node_path = "Button")]
     |                              ^^^^^^^^
...

If we were to parse this only as a list, the above wouldn't work, and instead we would need to only allow:
[Option 2]

// ..
  #[export(node_path = ("Class1"))]. // (list with only one entry)
// ...
}

I'm removing the [Option 1] for now though, since it's more in line with what is done in GDScript.

[Bromeon]

Do we need both? I guess the one with 1 argument is the common one?

[tuto193]

I left [Option 2] (#[export(node_path = ("Class1", ...))]) in. I think it's indeed more in line with what users could use, plus allows for more freedom/flexibility. Also, I will mark this as resolved unless you (or anyone else) is really against this, although I personally like the freedom plus it's the code is as easy to maintain as the other handle_list() branches :)

[Bromeon]

Please don't resolve conversations you still expect me to read, it's rather hard to locate them 🙂

But sounds good for now, we can always add the other syntax later.

[tuto193]

My bad 😅.

So I will leave it as is :)