fix(buttonGroup): Resolve the issue with the emission of buttonSelect…

[PlamenD95]

…ed on init

Closes #13802

The solution for the issue of calling the buttonSelected event on initialization includes removing the emission of the event from the selected setter in the button.directive.

In place of this functionality, an event listener is implemented in a new AfterContentInit hook, which emits the buttonSelected event only on user clicks.

A MutationObserver is added to check for changes in the DOM when the selected property of the button.directive is updated programmatically, so that the button group can update the styles of its buttons.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them