Remove the need for `null` at the end of `Promise<T>` and `GlobalEventHandlers` lambdas

[sdeleuze]

As seen in https://github.com/wasm-outbound-http-examples/kotlin/blob/main/browser/src/wasmJsMain/kotlin/Main.kt or some Spring + Kotlin/Wasm samples I have been working on, the requirement to return null as the latest statement of some lambdas in some APIs exposed by kotlinx-browser is a significant DevXP issue as it makes the Kotlin code IMO unnecessarily ugly and can block newcomers who will not understand that requirement. For example:

fun main() {
    window.fetch("https://httpbin.org/anything").then {
        if (it.ok) {
            it.text().then {
                println(it)
                null
            }
        }
        null
    }
}

It impacts at least Promise<T> based APIs and event handlers like those defined in GlobalEventHandlers, potentially other APIs too.

It would be a big DevXP improvement if that could be fixed.