feat(api): evaluate statusline string #16020

Adds API function `nvim_eval_statusline` to allow evaluating a
statusline string and obtaining information regarding it.

Closes https://github.com/neovim/neovim/issues/15849

src/nvim/api/keysets.lua

@@ -51,5 +51,12 @@ return {
   runtime = {
     "is_lua";
   };
+  eval_statusline = {
+    "winid";
+    "maxwidth";
+    "fillchar";
+    "highlights";
+    "use_tabline";
+  };
 }