Add modifiers for table captions

[frankieroberto]

This makes table captions consistent with fieldset legends and labels, in having modifiers which apply the typographic styles equivalent to headings.

Resolves issue 1405.

[CharlotteDowns]

This looks like a really good addition. I've left a few comments but you may have chosen to not do them intentionally based on the context. Let me know what you think and thanks again :).

[36degrees]

Do we need to update the Design System tables guidance to document these new classes?

[frankieroberto]

@36degrees I think it makes sense to mention these options on the table component page?

I'd be tempted to make the default example on that page using a larger caption size too.