Add support for searching tags and notebooks on mobile (Goto Anything on mobile)

Features
1

As per the documentation here Searching | Joplin the Goto Anything feature on desktop allows searching tags by prefixing the search with # and searching for notebooks by prefixing the search with @

The search button on the initial screen on the mobile app does not support either of these queries, but it could be extended to be a Goto Anything search, by allowing these prefixes to be used.

While it is possible to filter tag names on the Tags screen (which is accessible when a note is opened), this is not particularly convenient, and wont allow you to view the notes associated with that tag unless you go back to the initial screen and enter a search using the tag: prefix. Regarding notebooks, it is currently not possible on the mobile app to filter notebook names.

2

I had not considered that the search bar on the desktop app behaves the same as the global search on the mobile app, and the goto anything is actually a separate bit of functionality.

I don't see why goto anything and search could not be unified (there has been a request for this in the past), but if that is an issue then at least the goto anything functionality could be added to the mobile app. Maybe a button next to the search icon on the main note list, or add a 3 dots menu next to the search button, with the goto anything option under it.

Or if it's simpler, maybe just add a search button to the tags screen to be able to filter tags there