feat: allow children region to opt out of the default slot

[amanabiy]

The documenter marks any children region as the default slot (isDefault: true). Consumers such as the components website use that flag to render a "(default)" label and a generic "Default slot: Specify the content as a child of the component." note in the API docs.

For some components, such as top navigation, that annotation is misleading: children may be one of several supported ways to provide content rather than the single canonical default slot. Until now there was no way to express that, since isDefault was hardcoded to region.name === 'children'.

This adds an @isChildrenDefault JSDoc tag on a region so a component can opt its children slot out of the default-slot treatment, while keeping the existing behavior for everyone who does not use the tag.

Behavior:

• children, no tag -> isDefault: true (unchanged)
• children + @isChildrenDefault false -> isDefault: false
• any non-children region -> isDefault: false (unchanged)

Usage:

  export interface TopNavigationProps {
    /**
     * Specifies custom navigation content.
     * @displayname content * @isChildrenDefault false
     */
     children?: React.ReactNode;
   }

Adds the is-children-default-tag fixture and a test covering the opt-out case.

Issue #, if available: 1qZFXQh4o49s (pippin)

Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.12%. Comparing base (68cacaa) to head (b07a5cc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #118   +/-   ##
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  Coverage   95.11%   95.12%           
=======================================
  Files          11       11           
  Lines         512      513    +1     
  Branches      139      140    +1     
=======================================
+ Hits          487      488    +1     
  Misses         25       25           

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