Add support for workspace read requests

[msujew]

Related to microsoft/language-server-protocol#1264 (does not fully resolve it, since this PR does not include features to write into a file system - only read from it).

Adds support for the server to read files/directories/stat info from the client. Allows servers to "see" any registered remote/virtual file system files/directories via means of the LSP.

As indicated by microsoft/language-server-protocol#1264 (comment), I also thought it'd be best to start with read-only access to the file system. However, the new interfaces/types should be extendable enough to also add write requests if required later on.

Some questions/considerations:

• Right now, if the read/stat fails, the LSP returns null. Should it return a response error instead?
• I'm not entirely sure about the properties in FileSystemClientCapabilities. Does it make sense to expose each individual request type as an opt-in flag? Or is it enough to provide a read?: boolean flag (maybe extend this with a write?: boolean flag later on)?
• The FileType.unknown value results from the fact that vscode offers vscode.FileType.unknown as a possible value to return for FileStat.type. Should this be included in the protocol, or should stats/directory entries with this type simply be omitted?
• The LSP has been using folder instead of directory for the most part. Should this change align to this? I've simply used the same nomenclature as vscode does.

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[msujew]

@microsoft-github-policy-service agree