Add new mapping yaml validation

[JFisk42]

User Story

As a ReportStream developer I want to be notified when conditional operators are formatted incorrectly, so that I am immediately aware of my mistake.

Description/Use Case

For the HL7 -> FHIR mappings, if NOT NULL is provided for a conditional field instead of NOT_NULL there is no syntax error that appears. Would be a nice to have

Risks/Impacts/Considerations

With the mapping yaml validations, it would be nice to add more formatting checks as we come across them. Probably this task is best left until there are several to tackle all at once.

Dev Notes

One potential weird edge case here is that NULL by itself is a valid condition keyword but NOT NULL isn't. Because there is a space between NOT and NULL, the effort to work around that may not be worth the syntax highlighting.

Acceptance Criteria

• Our YAML validator highlights an issue when NOT NULL is entered into the condition line.

[MichaelEsuruoso]

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