Markdown parser: add property selection

[gorenje]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Add a selection of which property contains the markdown. This is analogous to the base64 node that also allows selection of the property on the msg object to be converted.

This can be just as useful for the markdown node. Also there is no special JSONata or flow context handling in the setting of the property, only properties on the msg object are valid.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team. --> https://discourse.nodered.org/t/markdown-node-select-property/99110
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality