Decouple Metadata as Source from the LSP miscellaneous files workspace

[jasonmalinowski]

Right now our LSP miscellaneous files workspace is coupled to metadata as source -- if a new file appears, it's up to the miscellaneous files workspace to forward it to the other workspace:

roslyn/src/LanguageServer/Protocol/Workspaces/LspMiscellaneousFilesWorkspace.cs

Lines 44 to 50 in f57fbdc

	if (metadataAsSourceFileService.TryAddDocumentToWorkspace(documentFilePath, container, out var documentId))
	{
	var metadataWorkspace = metadataAsSourceFileService.TryGetWorkspace();
	Contract.ThrowIfNull(metadataWorkspace);
	var document = metadataWorkspace.CurrentSolution.GetRequiredDocument(documentId);
	return document;
	}

I'm not really clear why we're doing this, other than borrowing the pattern from VS for Windows, where the Metadata as Source stuff is at a layer where it can't directly watch files being opened or closed. But in the LSP case, we could just generate the documents with a special URI from the start, and register the LSP workspace with those documents with those URIs, and just let things behave more naturally. I guess that's potentially bad from a perf perspective if you do hundreds/thousands of go to defs that require custom source...but maybe that's not really a concern?

This came up as we're building a more complicated Miscellaneous Files workspace for VS Code, since this bit of the implementation has to be duplicate when it doesn't really need to be.