Advertise use of markdown inside descriptions

[ssbarnea]

Many JSON Schemas make use of markdown inside their description fields as this is extremely useful for IDEs.

One notable example is vscode which uses the alternative markdownDescription field for that purpose. Still, almost nobody populates both fields, so the idea of having a different field name proves to be less than ideal.

That is why I want to propose a top level property such hasMarkdown: true that we can set at schema level that declares that descriptions and maybe other fields are using markdown.

Keep in mind that markdown is still valid as text so there is no major problem if a dump program would display description as plaintext when in fact it is markdown.