feat: Make toolbar-container selectable via shadow part

[fudom]

Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already include this feature request, without success.

Describe the Feature Request

<ion-toolbar>
  #shadow-root (open)
    <div class="toolbar-container" part="container"> <!-- add part -->

so we can select it via css

ion-toolbar::part(container) {
  flex-wrap: wrap;
  justify-content: end;
}