Add option for using npx instead of hardlinked eslint command

[JackBailey]

This PR adds the option useNpx, which will change the following command:
node node_modules/.bin/eslint **/*.js to npx eslint **/*.js, which doesn't break when using pnpm (Resolves #14).

This PR is currently functional, but I'm sure you'll want to edit the code to your liking and write some documentation for it.

Usage:

- name: Lint
  uses: jackbailey/action-eslint@6494c60dd93dbb092144ca58dbf0ca14e85e4574
  with:
      token: ${{ secrets.GITHUB_TOKEN }} # optional
      extensions: 'js'
      annotations: true
      use-npx: true

[Smef]

This would be helpful to have released. Could we get a 3.0.2 release with this included, please?