Pressing Tab inside a table should only select the next cell's content, not anything beyond that

[Andres-CT98]

Description:

When Pressing tab to get to the next cell in a table, #2718 & #2764, the selection should be limited to only the contents of the adjacent cell, or in the case of last cell, the next line.

Fix

When obtaining the cells' first and last child, their positions should be normalised using noramlizePos. After that, the offset values for the range should always be 0 for the starting node and the length of the final node for the end node. This asures that the new range is a valid one that always exists inside the cell.

Cases tested:

1. Create a table bigger than 2x2
2. Add text after the table
3. Go to the first cell and press Tab, this should select the next cell
4. Type a letter

Before: The text below the table is removed

After: The text below the table is intact

[JiuqingSong]

We can close this one now.