fix(api): merge split window config only on success

Problem: nvim_win_set_config may merge configs despite failing to configure a
split, and without applying necessary side-effects (like setting style=minimal
options). Plus, autocommands may apply a different config after the merge,
causing side-effects to apply for an outdated config.

Solution: merge configs last, only on success. Include fields only relevant to
splits. Properly set _cmdline_offset for splits.

Maybe better to disallow _cmdline_offset for splits instead, as the pum is
relative to cmdline_row anyway? (I didn't want to change behaviour too much)

Also use expect_unchanged in an unrelated test to quash a warning.

test/functional/ui/float_spec.lua

@@ -8942,17 +8942,7 @@ describe('float window', function()
           unchanged = true,
         }
       else
-        screen:expect([[
-          Ut enim ad minim veniam, quis nostrud             |
-          exercitation ullamco laboris nisi ut aliquip ex   |
-          ea co{2:test}{3:o consequat}. Duis aute irure dolor in    |
-          repre{3:henderit in vol}uptate velit esse cillum      |
-          dolor{2:popup}{3:fugi}{2:text}{3:ul}la pariatur. Excepteur sint   |
-          occaecat cupidatat non proident, sunt in culpa    |
-          qui officia deserunt mollit anim id est           |
-          laborum^.                                          |
-                                                            |
-        ]])
+        screen:expect_unchanged()
       end
       api.nvim_buf_set_lines(buf, 0, -1, true, test_data)