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/spellfile.c

@@ -5600,7 +5600,7 @@ static void init_spellfile(void)
                      && strstr(path_tail(fname), ".ascii.") != NULL)
                     ? "ascii"
                     : spell_enc()));
-      set_option_value_give_err("spellfile", CSTR_AS_OPTVAL(buf), OPT_LOCAL);
+      set_option_value_give_err(kOptSpellfile, CSTR_AS_OPTVAL(buf), OPT_LOCAL);
       break;
     }
     aspath = false;