Add #[wasm_bindgen(typescript_custom_section)] feature flag

[thorlucas]

This feature flag would allow types to be exported directly to wasm_bindgen's .d.ts files! Very useful.

Additionally, possibly these types can be auto cast to &JsValues using serde so they can be returned directly from functions.

[NyxCode]

I'm not at all familiar with that part of wasm_bindgen.
Maybe you could outline in detail what this issue is about or open a PR.

[NyxCode]

closing due to inactivity. If you still need this @thorlucas, please describe in detail what this issue is about.