PCH Excerpt Suggestions: Adds feature as a panel in the PCH sidebar

[vaurdan]

Description

This PR adds the Excerpt Suggestions feature to the PCH Sidebar, as it's own panel. The feature is still available as a replacement for the core excerpt panel, in the document sidebar.

There are some slight UI differences between the two panels, that are visible in the screenshots below.

Note: this work is based upon #2776, since it uses the work that was implemented there.

Motivation and context

• Make all the PCH features available in a single location
• Closes #2552

How has this been tested?

Tested locally

Screenshots (if appropriate)

UI in the document sidebar	UI in the PCH sidebar

Summary by CodeRabbit

• New Features

  • Introduced ExcerptSuggestionsPanel for improved excerpt suggestions with dynamic UI elements.
  • Added ExcerptDocumentSettingPanel for custom excerpt suggestions based on post type support.
  • Implemented a new "Excerpt Suggestions" panel in the sidebar, conditional on user permissions and post type.

• Bug Fixes

  • Streamlined rendering logic for success notices in the Smart Linking feature.

• Style

  • Enhanced styling for excerpt suggestions with new CSS classes for better layout and presentation.