fix(tab-bar): allow individually disabled tabs with enabled tab-bar

[MikeMatusz]

Fixes #707

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added/updated: Y
• Docs have been added/updated: Y
• Does this PR introduce a breaking change? N
• I have linked any related GitHub issues to be closed when this PR is merged? Y

Describe the new behavior?

Initially and when new tabs are added to the DOM, only override their disabled state if the tab-bar is disabled, otherwise respect the disabled state of the tabs itself.

Setting the disabled property of the tab-bar will still override the disabled state of all child tabs. Documentation has been updated to indicate that mixing the two methods of disabling tabs is not supported. In scenarios where tabs are always enabled/disabled as a group, the developer has the option of setting tab-bar disabled as a convenience. If there are scenarios where individual tabs can be disabled under separate conditions, then the developer should update the tab disabled properties exclusively.

Additional information

Had to update the test harness to update the disabled attribute rather than property. Lit's property binding always sets the property, and was explicitly setting the tab-bar disabled property to false, which is not the same as omitting the attribute.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[github-actions]

🚀 PR was released in v3.3.0 🚀