Add support for custom rendering of primitive cells

[b-gran]

Primitive schema types now accept a render function, which handles rendering of cell.
This render function enables use of <textarea> for longer input fields.

The render function accepts the following parameters:

function render(
  type: SchemaType, 
  value: string | number | boolean, 
  onChange: (value: string | number | boolean) => void
): React.ReactNode