Allow TextSearchProvider2 FileSearchProvider2 registration with "file" schema

[Elity]

Description

We have developed a extension Haystack Search to solve the issue of slow search in large projects in vscode, currently using a customized webview interface, which isn't very user-friendly. Our goal is to use TextSearchProvider2 and FileSearchProvider2 for the search functionality while maintaining vscode's native search interface for users.

But, it's not possible to re-register a TextSearchProvider2 with a "file" schema. This limitation forces extensions that need to implement custom search functionality to create a FileSystemProvider and transparently connect the current workspace to enable searching with TextSearchProvider2.

Proposal

Update the API to allow extensions to register TextSearchProvider2 with a "file" schema, at least for a limited set of extensions. This would enable more direct implementation of custom search functionality without the overhead of implementing a complete FileSystemProvider when only search capabilities are needed.