feat(options): default statusline expression #33036

Problem:
Default 'statusline' is implemented in C and not representable as
a statusline expression. This makes it hard for user configs/plugins to
extend it.

Solution:
- Change the default 'statusline' slightly to a statusline expression.
- Remove the C implementation.

test/functional/ui/multigrid_spec.lua

@@ -282,7 +282,7 @@ describe('ext_multigrid', function()
         screen:expect{grid=[[
         ## grid 1
           [4:----------]│[2:------------------------------------------]|*12
-          {11:<No Name]  }{12:[No Name]                                 }|
+          {11:[No Name]  }{12:[No Name]                                 }|
           [3:-----------------------------------------------------]|
         ## grid 2
                                                     |