Inner text node position should follow cursor position

[rparrett]

To reproduce:

• cargo run --example basic
• Type hello there
• Press left-arrow repeatedly until the cursor disappears

This reveals another issue with the janky zero-width cursor setup when left-arrow is pressed for the first time.

Roughly, we need to

• Get the relative cursor position from TextLayoutInfo
• Offset the inner text node so that the cursor is always visible

Note that this is going to be janky when there are spaces involved until Bevy 0.15 due to bevyengine/bevy#13063, and there's not much we can do about that.