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/lua/vim_spec.lua

@@ -4011,9 +4011,9 @@ stack traceback:
       ]]
       screen:expect [[
         19                            |
-        {2:[No Name] [+]  20,1         3%}|
+        {2:< Name] [+] 20,1            3%}|
         ^19                            |
-        {3:[No Name] [+]  20,1         3%}|
+        {3:< Name] [+] 20,1            3%}|
                                       |
       ]]
       exec_lua [[
@@ -4022,9 +4022,9 @@ stack traceback:
       ]]
       screen:expect [[
         99                            |
-        {2:[No Name] [+]  100,1       19%}|
+        {2:< Name] [+] 100,1          19%}|
         ^19                            |
-        {3:[No Name] [+]  20,1         3%}|
+        {3:< Name] [+] 20,1            3%}|
                                       |
       ]]
     end)