Add questionnaire and qItem as reserved variables

[FikriMilano]

Describe the Issue
Add questionnaire and qItem as reserved variables.

Note:
Based on this doc, I would argue that qItem is not supposed to be accessed via root level of any expression based extension. BUT still, I don't see the downsides of adding qItem as part of the reserved variables. It's simpler too.

The %qitem variable is defined as a shortcut to get to the Questionnaire.item that corresponds to the context QuestionnaireResponse.item. It is only valid for FHIRPath expressions defined within a Questionnaire item.
https://build.fhir.org/ig/HL7/sdc/expressions.html#fhirpath-supplements

Would you like to work on the issue?
Yes

[FikriMilano]

@jingtang10 @f-odhiambo this is a super small change

[FikriMilano]

@ndegwamartin

[MJ1998]

@FikriMilano

You mean that, based on doc, qItem can still be used in expression defined at questionnaire root level, right ?

[FikriMilano]

@MJ1998 I meant the opposite

[FikriMilano]

qItem is only valid when used in Questionnaire item

[MJ1998]

I mean the same. qItem should be a reserved item in Questionnaire items. But from definition, one should be able to use it as a variable name (not recognized as a reserved word) when declared in the questionnaire root extension(s), correct ?

But from code this will be considered as a reserved word for root level extensions as well.

[FikriMilano]

@MJ1998 correct and correct.
Let me know if you or anyone have objections to that.

[MJ1998]

Supporting this will also not be a big change but it's not needed currently.
I am fine with it. @jingtang10