Make the regular-table element accessible by default

Making grids and treegrids accessible is difficult, and requires a particular structure and set of annotations that conforms to the WAI-ARIA standard. It would be nice if we could make sure to provide consumers of `regular-table` with an accessible grid element by default.

refs for flat grids:

- https://www.w3.org/TR/wai-aria-practices/#grid
- https://www.w3.org/TR/wai-aria-practices/examples/grid/dataGrids.html

refs for treegrids:

- https://www.w3.org/TR/wai-aria-practices/#treegrid
- https://www.w3.org/TR/wai-aria-practices/examples/treegrid/treegrid-1.html

general refs:

- https://www.w3.org/TR/wai-aria-practices/#gridAndTableProperties