Able to define invalid XMLs with multiple possible root nodes

[madhukaw]

Description:
$Subject

Steps to reproduce:
Run the below sample.

import ballerina/io;

public function main() {
    xml x = xml `<name>Alex</name><age>30</age>`;
    io:println(x);
}

Output:

<name>Alex</name><age>30</age>

However, this is not a valid XML since there can be multiple possible root nodes.

Affected Versions:
SLBeta3

OS, DB, other environment details and versions:
MacOS

[MaryamZi]

The xml type in Ballerina represents all XML values where

An xml value is a sequence representing parsed XML, such as occurs in the content of an XML element.

See https://ballerina.io/spec/lang/master/#XML

An XML value can be a singleton of element, processing instruction, comment, and text or a sequence of multiple items. It is not required to be a valid XML document.