Implement registerFileSearchProvider/registerTextSearchProvider vscode proposed APIs

As I mentioned at the dev meeting- this is one example of a feature I'd use due to having a large repo with a customised VCS

Feature Description:

Support for custom URI schemes, specifically for file search (Quick Open) and text search (Find in Files)

https://github.com/Microsoft/vscode/blob/main/src/vs/vscode.proposed.d.ts#L249-L538 registerFileSearchProvider/registerTextSearchProvider

This can help for large repos, or to avoid checking out a repo/dependency but still add it to your workspace to navigate it.

One example of an extension supporting this, that can be used to test the implementation - https://github.com/eamodio/vscode-remotehub

Theia's current implementation uses ripgrep- this would be limited to file: URIs only and the code changed to match the interface TextSearchProvider. https://github.com/eclipse-theia/theia/blob/b74355fd3c133f8b2fe826b756b6a4851bc17961/packages/file-search/src/node/file-search-service-impl.ts

Or this could be swapped out for the MS implementation as it already matches the apis - subject to license- https://github.com/microsoft/vscode/blob/a699ffaee62010c4634d301da2bbdb7646b8d1da/src/vs/workbench/services/search/node/ripgrepSearchProvider.ts