generated d.ts definitions should use export declare class instead of export class

[sxlijin]

current behavior: NAPI currently generates class declarations in the d.ts file that look like this:

export class Foo {
  foo1(): void
  get foo2(): void
}

This is not valid in a class definition.

expected behavior: a class declaration:

export declare class Foo {
  foo1(): void
  get foo2(): void
}

Not totally sure where a PR to fix this would go, I glanced at https://github.com/napi-rs/napi-rs/blob/b2239fd880fa40fa98d206d8f31aec1bb8a0ce12/crates/backend/src/typegen/struct.rs but didn't see an obvious answer