refactor: use vim._with where possible

This mostly means replacing `nvim_buf_call` and `nvim_win_call` with
`vim._with`.

test/functional/ui/title_spec.lua

@@ -82,11 +82,11 @@ describe('title', function()
       end)
     end)
 
-    it('a Lua callback calling nvim_buf_call in a hidden buffer', function()
+    it('a Lua callback calling vim._with in a hidden buffer', function()
       exec_lua(string.format(
         [[
         vim.schedule(function()
-          vim.api.nvim_buf_call(%d, function() end)
+          vim._with({buf = %d}, function() end)
         end)
       ]],
         buf2