Validator API should throw an exception if inital value and intial value expression are both present.

[RaaziaTarique]

Is your feature request related to a problem? Please describe.
Since both initial.value and initialExpression generate values before the user has access to the form, it is not allowed to have both present on a Questionnaire.item.Validator API has to throw an exception if inital value and intial value expression are both present in the questionnaire.

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Additional context
Reference this url: http://build.fhir.org/ig/HL7/sdc/expressions.html

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