Add scoped name slot support for Flutter Widget Element.

[andycall]

For Flutter widget elements, we can add any HTML elements as children of widget elements:

Exp:

<flutter-button>
    <div>helloworld</div>
</flutter-button>

@override
  Widget build(BuildContext context, List<Widget> children) {
    return ConstrainedBox(
      constraints:
          BoxConstraints.tightFor(width: renderStyle.width.computedValue, height: renderStyle.height.computedValue),
      child: RoundedLoadingButton(
        borderRadius: 4,
          controller: controller, onPressed: () {
            _isLoading = true;
            dispatchEvent(MouseEvent(EVENT_CLICK, detail: 1, view: ownerDocument.defaultView));
      }, child: children.isNotEmpty ? children[0] : const SizedBox.shrink()),
    );
  }

But we can only put these HTML childrens in one place in our widget tree.

Now if we wants to implement a tab bar element with Flutter widgets, and we wants to use HTML tags to implement tab bar's header and body. We needs pass two chunk of HTML tags in to element, but we only have one place.

The WebComponent standard defined named slot can solve these problems: https://javascript.info/slots-composition#named-slots

And we can use for reference it to solve this problems.

Alternatives and Workarounds
No response