[KT-6300]: Stdlib: add findIs and firstIs method

[Komdosh]

There is a find and filterIs methods for iterables, but there is no method for finding one specific element in iterable.

For example:

val services = listOf<CommonInterface>(specificInterfaceA, specificInterfaceB, specificInterfaceC)

val specificService = services.findIs<SpecificInterfaceB>()

it is easy to implement, but it is still not in stdlib.

public inline fun <reified R> Iterable<*>.findIs(): R? {
    return find { element -> element is R } as R?
}

Forgotten for 11 years actually: KT-6300