`bevy_text`: use `u32`s for line and section indices (#24274)

# Objective

Parley uses `u32`s for its section indices, the conversion to `usize`
serves no purpose.
Line indices can be `u32`s too, text with more than `u32::MAX` lines is
infeasible.

## Solution

Change the section and line indices in `PostionedGlyph` and
`RunGeometry` to use `u32`s.