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/eval/funcs.c

@@ -7239,7 +7239,7 @@ int do_searchpair(const char *spat, const char *mpat, const char *epat, int dir,
     // If it's still empty it was changed and restored, need to restore in
     // the complicated way.
     if (*p_cpo == NUL) {
-      set_option_value_give_err("cpo", CSTR_AS_OPTVAL(save_cpo), 0);
+      set_option_value_give_err(kOptCpoptions, CSTR_AS_OPTVAL(save_cpo), 0);
     }
     free_string_option(save_cpo);
   }