`nvm_has` matches shell functions and aliases,
but downloads now run via `command`, which skips them
- a `curl` shell function with no curl binary on the PATH would select the curl path and fail with exit 127,
instead of falling back to an available wget executable.
The new `nvm_has_executable` helper resolves names the same way `command` does, so downloader selection and execution agree.
Add `--offline` flag to `nvm install` that resolves versions using only locally installed versions and cached downloads. No network calls are made.
New helper functions `nvm_ls_cached` and `nvm_offline_version` scan `$NVM_DIR/.cache/bin/` for previously downloaded tarballs.
In offline mode, `nvm_download_artifact` returns cached tarballs directly without checksum verification or download attempts.
The curl/wget requirement is skipped when `--offline` is set.
Supports `--lts` via locally stored LTS alias files.