[Fix] nvm_is_version_or_alias: reject uppercase LTS names in every locale

bash 3.2, macOS's `/bin/sh`, matches a bracket range like `[a-z]` by collation order in a UTF-8 locale,
which interleaves upper- and lowercase letters,
so `lts/Iron` from a symlinked `.nvmrc` was accepted there.

Match with `[:lower:]` instead.

nvm.sh

@@ -953,7 +953,7 @@ nvm_is_version_or_alias() {
     ;;
     lts/*)
       case "${1#lts/}" in
-        '' | *[!a-z]*) return 1 ;;
+        '' | *[![:lower:]]*) return 1 ;;
       esac
       return 0
     ;;