support newtype pattern in bindings

[ctaggart]

Motivation

It would be more type-safe if we could wrap primitives using the newtype pattern. The API would then become:

pub struct KeyCode(pub i32);

extern {
  # [ wasm_bindgen ( method , getter , js_name = keyCode ) ] 
  pub fn key_code(this: &IKeyboardEvent) -> KeyCode; 
}

Currently, KeyCode is defined as an alias and the enum values becomes constants in a separate type.

https://github.com/taggartsoftware/ts2rs/blob/6d357315c915758f1e5a3c9af2d7e94b203e9fab/monaco-editor/src/monaco_extern.rs#L219-L220

https://github.com/taggartsoftware/ts2rs/blob/6d357315c915758f1e5a3c9af2d7e94b203e9fab/monaco-editor/src/monaco_help.rs#L22-L171

Proposed Solution

Is there way to see if it is a single value struct & wrap it accordingly?

Alternatives

Leave as is, which is less type safe.

Additional Context

This would be helpful for all TypeScript enums translated to wasm-bindgen. taggartsoftware/ts2rs#4

[Pauan]

The user API would look something like this:

#[wasm_bindgen(transparent)]
pub struct KeyCode(i32);

This would apply a #[repr(transparent)] and generate the various trait impls.

[nasso]

Do we really need a wasm_bindgen(transparent) attribute if we already have #[repr(transparent)] (#1474)?