Use async find provider only for file scheme

[benibenj]

This pull request modifies the ExplorerView to utilize the async find provider exclusively for resources with the 'file' scheme. This change ensures that the find results provider is only instantiated when all roots support the file scheme.

fixes #230483

[gjsjohnmurray]

Instead of withdrawing Explorer async find for every scheme other than file I suggest you consider adding a temporary window-scoped setting allowing users to turn it off. This will provide a solution for users of FileSystemProviders for which there is not yet a FileSearchProvider, and should also placate users working with ordinary files (i.e. file scheme) who are unhappy with the loss of highlight-mode finding (edit: and folder-finding) in Explorer tree.

Perhaps make the setting default in such a way that the new async find provider is on for file scheme and off for all other schemes.

Ideally you'd be able to detect whether a FileSearchProvider for a given scheme is (a) available, (b) contributed but not yet initialized, or (c) not contributed. But if that's infeasible in time for a recovery release then the setting would suffice, plus it would bring a benefit for discontented file-scheme users.

[gjsjohnmurray]

Can you now use what @andreamah added in #230731?

[benibenj]

Not for the recovery release as it requires more changes which could be risky. If I use it, it will currently only work for file scheme and maybe remote, but not for extensions as the register late. Will think of a fix for insiders