Allow `#[wasm_bindgen(typescript_custom_section)]` to Include content from separate files using `include_str!`

[Colerar]

Motivation

The current implementation of #[wasm_bindgen(typescript_custom_section)] requires TypeScript code to be embedded directly as a string within the Rust source code. For example:

#[wasm_bindgen(typescript_custom_section)]
const TS_APPEND_CONTENT: &'static str = r#"
export type Coords = { "latitude": number, "longitude": number };
"#;

The motivation for this feature is to improve the developer experience by allowing TypeScript code to be written in separate files and included using include_str!. This would enable better syntax highlighting, improve code organization, and make TypeScript definitions easier to maintain.

Proposed Solution

The proposed feature is to enhance #[wasm_bindgen(typescript_custom_section)] so it can accept a string generated by include_str! from a separate file. This would allow developers to write TypeScript definitions in dedicated .ts files and include them in the Rust code at compile time.

#[wasm_bindgen(typescript_custom_section)]
const TS_APPEND_CONTENT: &'static str = include_str!("coords.ts");

[Colerar]

It seems to have been implemented:

wasm-bindgen/crates/typescript-tests/src/custom_section.rs

Line 9 in c35cc93

const TS_INTERFACE_EXPORT1: &'static str = include_str!("./custom_section_types.d.ts");

I'd like to add docs for this.