feat: use external application for approving and merging

[simoneb]

New implementation which uses the external app for merging: https://github.com/fastify/dependabot-merge-action-app

Also:

• bundling of action to avoid having to commit node_modules (and removing that from git), as suggested in the docs
• optional api-url action input to customize which API to hit (defaults to the hosted heroku app)
• linting
• use strict everywhere

Fixes #13

[mcollina]

lgtm

[bcomnes]

Can you clarify usage of the 2.0.0 version?

I need to enable the integration in addition to implementing the action? Or do I remove the action and just turn on the integration?

[simoneb]

@bcomnes see updated docs. The usage is unchanged but you need to install the companion GitHub App or host your own version