Fix setting of children main size in flex length resolving to follow the spec

[TimJentzsch]

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

Setting the main size of the children when resolving the flexible lengths does not seem to follow the specification: https://github.com/DioxusLabs/sprawl/blob/11701ee97d68b22a231cac081aab4308af136706/src/flexbox.rs#L519-L522

What solution would you like?

The implementation should be adjusted to follow the specification and refactored if necessary.

What alternative(s) have you considered?

If it works we could also leave it as-is.

Additional context

Originally discussed in #39.