Support tables in Markdown

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

Tables are a common requirement for markdown documents, but the WYSIWYG interface of the markdown widget doesn't support them.

**Describe the solution you'd like**

To create markdown tables visually in the Markdown Widget.

**Describe alternatives you've considered**

Creating them manually with raw markdown, this won't work for most users.

**Additional context**

Related to #1962

Start by reviewing the Markdown Widget's WYSIWYG interface and the discussion in related issue #1962. Identify how existing controls produce Markdown, then define the table-creation flow and confirm that it produces usable Markdown tables in the editor.

Written by the indexing model from the issue text.