`hidden_layout()` does not need to be a locally defined method

## What problem does this solve or what need does it fill?

`fn hidden_layout()` is currently defined within the scope of `fn compute_preliminary()`

## What solution would you like?

Move it to the main scope of `flexbox.rs`. It should be more easily testable that way.

## Additional context

Code: https://github.com/DioxusLabs/taffy/blob/main/src/flexbox.rs#L1721
