Add support for table navigation with up and down arrows

[jperedadnr]

Fixes #279

[abhinayagarwal]

• In a 2x2 table, if the cursor is in [1,1] pressing DOWN will take use to [2,1] and vice-versa on pressing UP. The same fails if the cursor is on the 2 column.
• If the table is longer than the viewport, a scrollbar is added to the RTA. Pressing DOWN then scrolls the scollbar instead of navigating throw the rows.

[abhinayagarwal]

Disregard my last comment, I had an old snapshot dependency.

Tested again and all looks good. Just one minor comment: when we go down, the cursor always is on the start of the text and when we go up it is always on the end of the text. Do we want to streamline the behavior or is this intended?

[jperedadnr]

It is intended. I've taken some inspiration from the Notes app (macOS).