Deprecate unaryPlus for Json array builder in favor of add.

[sandwwraith]

Mainly because of #418

There are three ways for it:

1. Do nothing.
2. Deprecate unaryPlus only for Number (and probably other primitives)
3. Deprecate all overloads, including one with JsonElement.

This PR implements 3), but I suggest considering 2) as a resolution, because using add everywhere seems verbose and overhead-ish to me (see diff of JsonArraySerializersTest). However, this will lead to inconsistency: JsonElement would be added via unaryPlus and other primitives via add. WDYT, @qwwdfsad ?

[qwwdfsad]

This is a subtle topic. I agree that add is too verbose and I am fine with an option when both add and + are present for every element type except Number to leave some options for ones who prefer consistency over verbosity.

But I think we can do better. Let's take a look at the JsonArraySerializersTest.

return jsonArray {
    add(1)
    add(JsonNull)
    add(jsonArray {
        add("nested literal")
    })
    add(jsonArray { })
    add(json {
        "key" to "value"
    })
}

what do we want to ideally see here? A consistent picture without a boilerplate.
Say, something like this:

return jsonArray {
    jsonLiteral(1)
    jsonNull // <- I have doubts about this one
    jsonArray {
        JsonLiteral("nested literal")
    }
    jsonArray { } 
    jsonObject {
        "key" to "value"
    }
}

what do you think about that approach? I tend to think that + and add are really unnecessary here and the only choice we have to make is whether these builders should look like plain json elements JsonArray { ... } or like functions jsonArray { ... }

[qwwdfsad]

(obligatory comment for GitHub to finish the review)

[sandwwraith]

It looks nice but seems a bit overkill, since this DSL would not be used often (and when it is used, I assume, it would contain one-two calls in a for loop). There is also a problem which can cause a mistake you've already made in the example: jsonArray { jsonLiteral(1) } works, but jsonArray { JsonLiteral(1) } (notice capital letter) does not.

[qwwdfsad]

It looks nice but seems a bit overkill, since this DSL would not be used often

I am not sure this is a valid point. Either we should provide a best-effort DSL or nothing at all.

There is also a problem which can cause

Then probably it's worth to make builder extensions undistinguishable from constructors?

[sandwwraith]

Closing in favor of #794