Add file references provider

[mjbvz]

I propose a new file references API that lets an extension tell VS Code about references to a given file. Possible use cases:

• Listing file references Find usages (find all file references / find all folder references) #66162
• Safe delete Safe File Delete #52844

#21018 was somewhat related but for more generic relationships between files, not just references

JS/TS implements file references with a custom solution today. I feel it makes sense to lift this into a proper VS Code api so that we can provide a consistent experience for across languages

[mjbvz]

Proposed API would be quite simple:

export interface FileReferenceProvider {
	provideFileReferences(resource: vscode.Uri,  token: CancellationToken): ProviderResult<Location[]>;
}

export function registerFileReferenceProvider(selector: DocumentSelector, provider: FileReferenceProvider): Disposable;

I'm using a Location as the return type so the providers can return the specific line number of the reference

[mjbvz]

Added custom implementation to markdown with #146267

[dummdidumm]

A note regarding the current custom implementation (which I also noted in #151896): Right now "find file references" is implemented by temporarily switching the user config from "peek" to "view" to force-open the references view, which creates an empty vscode setting folder/file in the workplace if none exists already, which feels buggy.