Add DSL-like initializers for document and element

[sgade]

I recently looked at a bit of XML-handling code of mine and thought that the API could be improved to be more ergonomic using @resultBuilders.

This introduces two @resultBuilder-based builders: AEXMLDocumentBuilder and AEXMLElementBuilder. The main difference is that the first produces an AEXMLDocument and only accepts a singular AEXMLElement as its root. The latter produces an array of AEXMLElements.

Both can be used in conjunction to create new hierarchies of elements easily and quickly. Instead of creating child elements before adding them to their parents, they can be created in code in a way that more closely resembles the actual XML structure in the resulting document.

An example use from code that I plan to ship into production is the following. Note how the structure of the document is easily defined and resembles the XML string that would be generated from it.
I added the document builder to the function but I could as well have just wrapped the root element in a AEXMLDocument {} without it.

    @AEXMLDocumentBuilder
    private func buildSoapEnvelope(
        for action: String,
        in serviceType: String,
        with parameters: [String: String] = [:]
    ) -> AEXMLDocument {
        AEXMLElement("s:Envelope", attributes: [
            "xmlns:s": "http://schemas.xmlsoap.org/soap/envelope/",
            "s:encodingStyle": "http://schemas.xmlsoap.org/soap/encoding/"
        ]) {
            AEXMLElement("s:Body") {
                AEXMLElement("s:\(action)", attributes: [
                    "xmlns:u": serviceType
                ]) {

                    for parameter in parameters {
                        AEXMLElement(
                            name: parameter.key,
                            value: parameter.value
                        )
                    }

                }
            }
        }
    }

I think this API is very flexible and adds to the library's readability. It doesn't take away any existing APIs so they remain the same. Would love your feedback. Just let me know what you think about this addition and if I should adjust anything.

(Btw, I have been using your library for years and I am super happy with it! Thank you for providing this to the community!)

[tadija]

Hi @sgade, thank you for this nice contribution and kind words! This change is now merged on develop and will roll out with the next release soon.