LSP: Feature/add workspace folders (#12638)

* First implementation of workspace folders
* Add completion for current directory
* Add tracking of workspace folders
* Add workspace folder listing
* Add checks on adding/removing workspaces
* Add appropriate initialization options
* Add documentation
* Make workspaceFolders available wherever client is

runtime/lua/vim/lsp/util.lua

@@ -1314,6 +1314,9 @@ function M.make_text_document_params()
   return { uri = vim.uri_from_bufnr(0) }
 end
 
+function M.make_workspace_params(added, removed)
+  return { event = { added = added; removed = removed; } }
+end
 --- Returns visual width of tabstop.
 ---
 --@see |softtabstop|