The `Measure` trait can't be implemented without importing taffy

## What problem does this solve or what need does it fill?

The `Measure` trait is `pub`:
```rust
pub trait Measure: Send + Sync + 'static {
fn measure(&mut self, measure_args: MeasureArgs<'_>, style: &taffy::Style) -> Vec2;
}
```
but we don't re-export `taffy::Style`, so it can't be implemented without adding a taffy dependency.

## What solution would you like?

We can re-export `Style` but that's not great, there are also a bunch of helper functions that are needed really to work with its types and some of the fields aren't used by bevy.

Start at the public Measure trait and inspect its measure signature, especially the dependency on taffy::Style and the helper functions needed around its types. Compare the trait's required style data with the fields Bevy actually uses. Done means the API direction is decided and a Measure implementation no longer requires consumers to add a direct taffy dependency.

Written by the indexing model from the issue text.