Change `Flexible`'s default `FlexFit`

Previously, `Flexible` defaulted to `FlexFit.tight`, which forced the child to
expand to fill the available space. Now, `Flexible` defaults to
`FlexFit.loose`, which does not force the child to expand to fill the available
space.

If you want the child to expand to fill the available space, consider using
`Expanded` instead.

Fixes #5169