Fix support markdown extension

[patrick-werner]

This pull request improves the handling of FHIR rendering-markdown extensions by adding support for both the correct valueMarkdown and the legacy valueString keys during import and export. It ensures that the original format is preserved and round-tripped correctly, and adds tests for this behavior.

FHIR rendering-markdown support improvements:

• Added a utility function getMarkdownExtensionValue in internal-utils.js to extract markdown text from either valueMarkdown or valueString, ensuring compatibility with both new and legacy FHIR data.
• Updated SDC import logic in sdc-import-common.js to use the new utility, store the original markdown value and its type, and pass these through to the export logic. This preserves whether the source used valueMarkdown or valueString. [1] [2] [3]
• Updated SDC export logic in sdc-export-common.js to use the correct key (valueMarkdown or valueString) based on the original import, ensuring round-trip fidelity. [1] [2]

Testing:

• Added a new test questionnaire JSON file using valueMarkdown for rendering-markdown, and a corresponding Karma test to verify correct import and export handling. [1] [2]

[patrick-werner]

fixes #191