Add a "Find in post" / text search within the block editor

What problem does this address?

Add an in-editor "Find" (text search) function so users can jump to a specific
word or section within a post they're editing, rather than manually scrolling.

Problem it solves

There is currently no way to search for text inside a post while editing it in
the app. For longform posts this means a lot of scrolling up and down to locate
a section. A user reported switching from the phone to a laptop specifically
because the desktop browser's find function lets them locate a section quickly,
whereas the app does not offer an equivalent. That device-switching workflow is
also what led to their edits being overwritten by an older version, so an
in-editor find would remove the need to switch devices for this reason.

Proposed behavior

• A "Find" action available while editing a post (e.g. from the editor overflow
  menu or a search icon).
• Typing a term highlights/jumps to matches within the post content.
• Optional: next/previous match navigation for multiple hits.

Customer request on 11556005-zen

Start at the block editor editing screen and inspect how its overflow menu or other editor actions are exposed. Define the supported search interaction, including match highlighting or jumping and whether next/previous navigation is included; done means users can find text within the post without leaving the editor.