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/testnvim.lua

@@ -24,6 +24,7 @@ M.nvim_set = (
   'set shortmess+=IS background=light noswapfile noautoindent startofline'
   .. ' laststatus=1 undodir=. directory=. viewdir=. backupdir=.'
   .. ' belloff= wildoptions-=pum joinspaces noshowcmd noruler nomore redrawdebug=invalid'
+  .. [[ statusline=%<%f\ %{%nvim_eval_statusline('%h%w%m%r',\ {'maxwidth':\ 30}).width\ >\ 0\ ?\ '%h%w%m%r\ '\ :\ ''%}%=%{%\ &showcmdloc\ ==\ 'statusline'\ ?\ '%-10.S\ '\ :\ ''\ %}%{%\ exists('b:keymap_name')\ ?\ '<'..b:keymap_name..'>\ '\ :\ ''\ %}%{%\ &ruler\ ?\ (\ &rulerformat\ ==\ ''\ ?\ '%-14.(%l,%c%V%)\ %P'\ :\ &rulerformat\ )\ :\ ''\ %}]]
 )
 M.nvim_argv = {
   M.nvim_prog,