Add a new `UiSchema` `ui:enableMarkdownInHelp` flag

[TheOneTheOnlyJJ]

Prerequisites

• [x] I have read the documentation

What theme are you using?

core

Is your feature request related to a problem? Please describe.

This issue is a branch-off from #4527, which was closed as completed, but discussions about this particular flag remained unaddressed.

As of v5, the Help fields render markdown sporadically, depending on the theme being used, as a result of using the ui:enableMarkdownInDescription flag. This is undesirable behaviour.

Describe the solution you'd like

A new UiSchema ui:enableMarkdownInHelp flag that would enable similar behaviour as with ui:enableMarkdownInDescription.

Describe alternatives you've considered

Making ui:enableMarkdownInDescription enable markdown rendering in Help fields as well. This would be undesirable for some users. We should strive towards more granular control, thus, a proper new flag should handle this separate concern.