Add support for element extension: regex

[fredhersch]

Is your feature request related to a problem? Please describe.
As a questionnaire developer I can add in regex expressions for item validation

Describe the solution you'd like

• Given a regex expression within a valid FHIR Questionnaire, use the expression to validate the input field when moving to the next field
• If there is a validation error, display the error message in-line (as per issue)
• TODO: Review current Questionnaire for the validation error text

Additional context
See example for a postal code regex expression within the HIV Case Reporting questionnaire

        <extension url="http://hl7.org/fhir/StructureDefinition/regex">
          <valueString value="[A-Z][0-9][A-Z] [0-9][A-Z][0-9]"/>
        </extension>

Would you like to work on the issue?
Happy to support