[Feature] Add standard macOS function "Use Selection for Find"

[nk9]

One of the standard macOS search commands is "Use selection for find". This takes the selected text and initiates a search for it. It's equivalent to Cmd-C, Cmd-F, Cmd-V. It does this without opening the find UI; instead, it directly uses the find pasteboard (modern API). This means it's less disruptive than Cmd-F: there's no unnecessary UI and the search query also doesn't overwrite the contents of the general pasteboard.

This menu item is missing in Typora. And even worse, the shortcut which is usually assigned to this function, Cmd-E, instead selects the styled scope. 😦

At the very least, please implement the "Use selection for find" menu item. And ideally, please also change the keyboard command (at least on Mac) so that Cmd-E does what users expect based on every other Mac app.

Note that native macOS apps use the find pasteboard to allow quick searching between apps. See more in #6128.