Hide extension button when embedded in an IDE (#9956)

* Hide the extension settings action when DevTools is running in an IDE.

Fixes https://github.com/flutter/devtools/issues/8507

* Ensure that the extensions action is displayed when showing an
extension on screen, even when embedded in an IDE.

* Replace https://flutter.dev/ with a data placeholder in
_controller_web.dart to prevent browser X-Frame-Options
iframe errors during local debugging.

* Add debugging devtools extensions skill

* add to release notes

* format

* Apply suggestions from code review

Co-authored-by: Kenzie Davisson <43759233+kenzieschmoll@users.noreply.github.com>

* Use OS-based browser opening commands

* Refactor HTML into a function

* remove embedMode information from skill, since it's unrelated

* Extract conditionals into local variable

* Use isEmbedded() to ensure that  embedMode=One or embedMode=Many are allowed

* format

---------

Co-authored-by: Kenzie Davisson <43759233+kenzieschmoll@users.noreply.github.com>