Add more support for JS primitives and common objects

[kgar]

Numbers, booleans, and Dates currently do not render when passed through the getMarkdownString() function. They return as empty strings.

Any primitive or common JS object that has a reasonable string equivalent should be able to be rendered.

Note: these primitives should have renderers of their own which can be overridden.

Add renderers for

• Date
  • Default behavior: .toISOString()
• boolean
  • Default behavior: .toString()
• number
  • Default behavior: .toString()
• bigint
  • Default behavior: .toString()
• null
  • Default behavior: return empty string
• undefined
  • Default behavior: return empty string

If we find that more are needed, we can put out another GitHub issue to get more added.