Add `.ex-table` class

Add an `.ex-table` class that allows the user to use a table as a set of subexamples, rather than a list of subexamples.

Include an optional `.implicit-header` class which allows the user to include a table header but hide it visually.

Here's one implementation:

https://github.com/dwhieb/Sitimaxa.info/blob/main/src/classes/ex-table.css