add a way to generate `tsconfig.json` files and change the `"target"` property

Feature description

I want to use Map<K, V> , but my code editor generates an error because the default "target" is "es5" and Map was introduced in es6

Basic example

#[wasm_bindgen(typescript_custom_section)]
const TS_APPEND_CONTENT: &'static str = "
type XYZ<K, V> = Map<K, V>
";