[Fix] `nvm_normalize_lts`: only reject uppercase for LTS names, not regular aliases

The lowercase check was in the `*` catch-all branch of the `case` statement, rejecting any alias name with uppercase characters.
This prevented creating or reading uppercase aliases like `TESTY`.
The check should only apply to `lts/*` patterns, since LTS codenames are always lowercase.

Fixes #3764.
Bug introduced in https://github.com/nvm-sh/nvm/commit/9fb9dec710e2c8d3aecb24c2a7c9f9fb45b1435b as part of fixing #3417.