List the available syntaxes in the Content macro's parameters modal

The content macro also gives a technical message when the syntax is not available: {{content syntax="markdown/1.2"}}x{{/content}} renders "Failed to execute the [content] macro. Cause: [Can't find descriptor for the component with type [interface org.xwiki.rendering.parser.Parser] and hint [markdown/1.2]]", which names neither the parameter nor the syntaxes that are available. The list of available syntaxes asked here would also let the macro refuse an unavailable one with an error that names the parameter and lists the available syntaxes.

This KO comes from the WYSIWYG usability audit of the macros, under the criterion E2 errors of its usability criteria. It is only partly covered here: the same KO also reports invalid values covered by XCOMMONS-3847 and XRENDERING-823.