nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.
When `.nvmrc` or alias files contained comments (lines with `#`),
the `#` character could end up in the search pattern passed to sed,
causing "unterminated regular expression" errors because `#` is
used as the sed address delimiter.
This commit fixes the issue in two places:
1. `nvm_alias`: Strip comments from alias file contents before
returning them, and trim trailing whitespace
2. `nvm_ls`: Escape `#` characters in SEARCH_PATTERN so they're
treated as literal characters in the sed address
Fixes#3761
