Add support for rowspan on tables

[Bamboo72]

Follow-up to #51 / #257, which added nested columns and grouped header rows.

As a developer using Atomic Elements, I want the option to create tables where the headers have been "split" so that I can show subcolumns, but I don't want any extra blank cells.

┌─────────────┬─────────┐        ┌─────────────┬─────────┐
│    Name     │ (empty) │        │    Name     │         │
├──────┬──────┼─────────┤   vs   ├──────┬──────┤   Age   │
│First │ Last │   Age   │        │First │ Last │         │
└──────┴──────┴─────────┘        └──────┴──────┴─────────┘

Currently it is possible to work around this by styling the blank cells, but it has the downside of screen readers getting confused about what to read. (Note: this is something that needs to be tested after #257 is merged because it might solve the problem)

Additionally a single taller cell reads as a more polished design.

Acceptance

• A column shallower than the tallest group spans the full header height as one cell, with no placeholder
• Header keyboard navigation still works across the grouped header
• Summed colspan per header row equals the column count, including after the column changes
• Grouped headers still work with selectionMode set