feature(markdown) Provide same header options as rich-text

[yellis]

Is your feature request related to a problem? Please describe.
Rich text mode allows headers 1-3. Markdown only allows H1.

Describe the solution you'd like

• Add H4-6 to rt mode
• Expand options in md mode to be equal to those in rt mode

MSE post

[AncientSwordRage]

Probably worth clarifying that it's the Headers menu option in Markdown Mode that only allows H1, the specification allows H1-6?