main: Extract normal_check from main_loop

The new function contains logic that must be executed after handling input in
normal mode and also before the first main loop iteration. Also rename
`main_loop` to `normal_enter` and move it to normal.c

src/nvim/ex_getln.c

@@ -4943,7 +4943,7 @@ static int ex_window(void)
    * Call the main loop until <CR> or CTRL-C is typed.
    */
   cmdwin_result = 0;
-  main_loop(TRUE, FALSE);
+  normal_enter(true, false);
 
   RedrawingDisabled = i;