refactor(options): reduce findoption() usage

Problem: Many places in the code use `findoption()` to access an option using its name, even if the option index is available. This is very slow because it requires looping through the options array over and over.

Solution: Use option index instead of name wherever possible. Also introduce an `OptIndex` enum which contains the index for every option as enum constants, this eliminates the need to pass static option names as strings.

src/nvim/ui.c

@@ -235,7 +235,7 @@ void ui_refresh(void)
   p_lz = save_p_lz;
 
   if (ext_widgets[kUIMessages]) {
-    set_option_value("cmdheight", NUMBER_OPTVAL(0), 0);
+    set_option_value(kOptCmdheight, NUMBER_OPTVAL(0), 0);
     command_height();
   }
   ui_mode_info_set();