When using pin_all_from, filenames containing "js" as a substring
(like "foo.jszip.js" or "bar.jsmin.js") were having the substring
incorrectly removed, resulting in malformed module names like
"foozip" and "barmin" instead of "foo.jszip" and "bar.jsmin".

Fixed the logic to only remove the file extension from the end
of the filename, preserving any "js" substrings that appear
elsewhere in the name.

Fixes #282

Fix `pin_all_from` incorrectly removing "js" substring from filenames

Test with Ruby 4.0 and Rails 8.1

Packages with nested paths (e.g., "photoswipe/lightbox" or
"@github/webauthn-json/browser-ponyfill") were not being correctly
identified, leading to issues when checking for outdated versions. This
update modifies the package name extraction logic to correctly handle
such cases.

Fixes #321,

Handle package names with nested paths in import maps