feat: Don't transform entrypoints prefixed with `public:`

[aklinker1]

This closes #167. If you add public: in front of an entrypoint in your manifest, the plugin will render it to the final manifest without processing the file.

For example, if you want to load jquery into a content script, and have downloaded a version locally:

public/
  vendor/
    jquery.min.js
src/
  my/
    content-script.ts

{
  // ...
  "content_scripts": [
    {
      "matches": [...],
      "js": ["public:vendor/jquery.min.js", "src/my/content-script.ts"],
    }
  ]
}

If possible, I would recommend just using the NPM modules, but sometimes it's easier to just keep using a version from a CDN.