feat: add layout improvements behind feature flag

[sissbruecker]

Adds the following improvements to vaadin-horizontal-layout and vaadin-vertical-layout:

• When using setFullWidth, setFullHeight or setFullSize, users typically expect those children to take up the remaining space in the layout, rather than have exactly the same size as the layout itself. In that case apply flex: 1 to make them take up the remaining space.
• Nested layouts that are supposed to take up the remaining space may not shrink because the size of their contents is larger than the remaining space in the layout. So specifically for other layout components, also apply min-width: 0 to allow them to shrink to the remaining available space.

The improvements are behind a feature flag as they may alter existing layouts, so for now we allow users to try them out.

Part of vaadin/flow-components#6998

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sissbruecker]

We decided to go with this approach instead customizing flex using a custom CSS property. Using a custom property would have meant to apply this rule to all children, which could then result in overwriting defaults that a component might have defined in their shadow root styles. That is because a rule like flex: var(--_layout-flex) reverts the flex value to initial if --_layout-flex is not defined.

[sissbruecker]

Similarly, changing the min size on all children is not desirable. For example, for grid we explicitly added a min-height in the shadow root styles, which should not be overridden by the layout. Other components have an explicit min size as well. As the problem with children not being able to shrink due to their content mostly involves nested layouts we chose to limit this to these two components for now.