Desktop app: Add in-editor search

### What

Add an option to search for text inside a specific post in the editor - as can be done by pressing Ctrl/Cmd-F in a browser

### Why

With longer posts and pages, this would allow people to quickly find specific text they need to edit.

I can see this being useful to some people in the Reader view in the app as well.

Requested in https://wordpress.com/forums/topic/search-function-for-the-deskop-app/?view=all

### How

_No response_

No file, test, or code entry point is named. Start by locating the desktop editor's post view and checking whether the requested search should also apply to Reader view; done means Ctrl/Cmd-F can find text within the selected post and supports the stated editing use case.

Written by the indexing model from the issue text.