New utility render-functions for RenderContext

[jamowei]

This PR and PR #783 added new utility render-functions inside RenderContext.

The renderIf(predicate: (V) -> Boolean) function only renders the given content, when the given predicate returns true for the value inside the Flow<V>.

val store = storeOf<String>("foo")
render {
    store.data.renderIf(String::isNotBlank) {
        p { +it }
    }
}

The renderNotNull() function only renders the given content, when the value inside the Flow<V?> is not null.

val store = storeOf<String?>(null)
render {
    store.data.renderNotNull {
        p { +it }
    }
}

The renderIs(klass: KClass<W>) function only renders the given content, when the value inside the Flow<V> is a type of klass.

interface I
class A: I
class B: I

val store = storeOf<I>(A())
render {
    store.data.renderIs(A::class) {
        p { +"A" }
    }
}

Extends the PR #783 from @serras. Thanks for your work!