Implement workspace/willRenameFiles notification

[retailcoder]

Specifications (LSP)

The will rename files request is sent from the client to the server before files are actually renamed as long as the rename is triggered from within the client either by a user action or by applying a workspace edit. The request can return a WorkspaceEdit which will be applied to workspace before the files are renamed. Please note that clients might drop results if computing the edit took too long or if a server constantly fails on this request. This is done to keep renames fast and reliable.

The handler for this request should evaluate whether workspace edits should be applied before the files are created, and promptly return them if that is the case. The cancellation token should be checked regularly to throw if cancellation was requested.

A first iteration is probably perfectly fine with simply returning null.

NOTE: in LSP "files" refers to both files and folders.