feat: Table cell/column widths in API

[matthewlipski]

You can already resize table cells in BlockNote, but these use premade TipTap nodes and so their widths are not really accessible to get/set via the BlockNote API. This PR modifies the typing of TableContent so that you can now read and update table cell widths. Additionally, it fixes a small bug that was preventing the cell widths from being parsed correctly.

The changes I made are based on #846, but I don't have push rights to that remote repo so this PR replaces that one.

Closes #655