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/runtime.c

@@ -1061,7 +1061,7 @@ static int add_pack_dir_to_rtp(char *fname, bool is_pack)
     xstrlcat(new_rtp, afterdir, new_rtp_capacity);
   }
 
-  set_option_value_give_err("rtp", CSTR_AS_OPTVAL(new_rtp), 0);
+  set_option_value_give_err(kOptRuntimepath, CSTR_AS_OPTVAL(new_rtp), 0);
   xfree(new_rtp);
   retval = OK;