Add simple support for workspacefolders

Add simple support for workspacefolders

Close https://github.com/joaotavora/eglot/issues/893.

Clients can support workspaceFolders since LSP 3.6.  rootUri and
rootPath are deprecated.  Dynamic changes in folders are not
supported, i.e., this patch does not implement
workspace/didChangeWorkspaceFolders.

* eglot.el (eglot-client-capabilities): Add capability
`workspaceFolders'.
(eglot-workspace-folders): New cl-defgeneric.
(eglot--connect): Add workspaceFolders to initializeParams.
(eglot-handle-request workspace/workspaceFolders): New cl-defmethod.