[Fix] prevent sed errors when pattern contains `#`

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