nvim: configure sourcekit-lsp

Need to work around a bug in  nvim-lspconfig:
  neovim/nvim-lspconfig#3264

nvim/init.lua

@@ -312,6 +312,23 @@ require("lazy").setup({
           capabilities = capabilities,
         }
       end
+
+      lspconfig.sourcekit.setup {
+        filetypes = { 'swift', 'c', 'cpp', 'objective-c', 'objc', 'objective-cpp' },
+        on_init = function(client, initialization_result)
+          if client.server_capabilities then
+            client.server_capabilities.semanticTokensProvider = nil
+          end
+        end,
+        get_language_id = function(_, ftype)
+          if ftype == "objc" then
+            return "objective-c"
+          elseif ftype == "objcpp" then
+            return "objective-cpp"
+          end
+          return ftype
+        end,
+      }
     end
   },