egui_extras: table cell dividers

**Is your feature request related to a problem? Please describe.** Kinda. I want to make cell dividers but adding separators makes the text *literally disappear*, which is a problem on its own.

**Describe the solution you'd like**
I want the ability to add cell dividers (both between rows and columns) and a border around the edge of the table. All of which should be customizable by color and size in pixels.

**Describe alternatives you've considered**
I tried using separators (e.g., `ui.add(Separator::default().vertical());` on both sides of the label in each row/column) which made the text disappear.

I haven't seen any issues for this so I'm making this one

Start by locating the egui_extras table implementation and reviewing how Separator is handled in table cells. Define the behavior for row and column dividers and the outer border, including customizable color and pixel size. Done means tables can render all three border types without making cell text disappear.

Written by the indexing model from the issue text.