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.

src/nvim/options.lua

@@ -8541,7 +8541,14 @@ local options = {
     {
       abbreviation = 'stl',
       cb = 'did_set_statusline',
-      defaults = '',
+      defaults = table.concat({
+        '%<',
+        '%f %h%w%m%r ',
+        '%=',
+        "%{% &showcmdloc == 'statusline' ? '%-10.S ' : '' %}",
+        "%{% exists('b:keymap_name') ? '<'..b:keymap_name..'> ' : '' %}",
+        "%{% &ruler ? ( &rulerformat == '' ? '%-14.(%l,%c%V%) %P' : &rulerformat ) : '' %}",
+      }),
       desc = [=[
         When non-empty, this option determines the content of the status line.
         Also see |status-line|.