Allow Kotlin's null literal in JSON DSL

[lukellmann]

Right now you can not really use Kotlin's null literal with the JSON DSL:

val obj = buildJsonObject {
    put("string", "string")
    put("boolean", true)
    put("number", 1)

    // Error: Overload resolution ambiguity. All these functions match.
    // - public fun JsonObjectBuilder.put(key: String, value: Boolean?): JsonElement? defined in kotlinx.serialization.json
    // - public fun JsonObjectBuilder.put(key: String, value: Number?): JsonElement? defined in kotlinx.serialization.json
    // - public fun JsonObjectBuilder.put(key: String, value: String?): JsonElement? defined in kotlinx.serialization.json
    put("null", null)
}

val arr = buildJsonArray {
    add("string")
    add(true)
    add(1)

    // Error: Overload resolution ambiguity. All these functions match.
    // - public fun JsonArrayBuilder.add(value: Boolean?): Boolean defined in kotlinx.serialization.json
    // - public fun JsonArrayBuilder.add(value: Number?): Boolean defined in kotlinx.serialization.json
    // - public fun JsonArrayBuilder.add(value: String?): Boolean defined in kotlinx.serialization.json
    add(null)
}

val p0 = JsonPrimitive("string")
val p1 = JsonPrimitive(true)
val p2 = JsonPrimitive(1)

// Error: Overload resolution ambiguity. All these functions match.
// - public fun JsonPrimitive(value: Boolean?): JsonPrimitive defined in kotlinx.serialization.json
// - public fun JsonPrimitive(value: Number?): JsonPrimitive defined in kotlinx.serialization.json
// - public fun JsonPrimitive(value: String?): JsonPrimitive defined in kotlinx.serialization.json
val p3 = JsonPrimitive(null)

All of these errors could be resolved by using a cast like null as String? or by using JsonNull directly.
However, this is somewhat irritating as other primitives that are supported by JSON (numbers, strings, true and false) can be used directly while null can't.

To resolve the overload resolution ambiguity and allow use of the null literal in the JSON DSL, this PR adds overloads for the JsonObjectBuilder.put(), JsonArrayBuilder.add() and JsonPrimitve() functions that take an argument of type Nothing?.

[lukellmann]

I haven't added @ExperimentalSerializationApi to the new functions yet, but I'm supposed to do this, right?

[lukellmann]

I haven't added @ExperimentalSerializationApi to the new functions yet, but I'm supposed to do this, right?

I added the annotations now.

[sandwwraith]

Sorry for the long delay, your PR slipped out of my attention. You were completely right with @ExperimentalSerializationApi. I think we can merge this after you address the comments

[sandwwraith]

Are you sure that this function is needed? It's much easier here to use JsonNull instead.

[lukellmann]

It's not strictly needed, but it feels more consistent to me.
But it's fine for me if you think this one is unnecessary and should be removed.

[sandwwraith]

Well, maybe it would be easier to discover. Then it's better to use doc style as in the other functions:

/**
* Creates [JsonNull].
*/

[lukellmann]

I adjusted the doc style 👍

[sandwwraith]

Thank you for your efforts!