[Improvement]: Support step extension in xml step expression

[poorna2152]

Description

xml step expression can have step extension which can be a method invocation, filter expression, indexed based access or a combination of these. https://ballerina.io/spec/lang/master/#xml-step-expr
eg:

xml x1 = xml
            `<item><!--comment--><name>T-shirt</name><price>19.99</price><brand><name>nike</name><local>no</local></brand></item>`;
_ = x1/**/<name|iname>[i].<name>;

Xml step extension is not properly implemented and the following are created to track the issues involving step expression.

• Align method invocation behavior in XML navigation/step expressions #21522
• Method invocations in XML step expressions are not aligned with the spec. #22136
• Index access within XML step expressions are disallowed. #22138
• Method invocations are not yet supported within XML navigation expressions error is emitted when xml navigation expression is used as a argument of a function invocatin. #27803

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-> Compilation

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