Add renovate

[LecrisUT]

Here is a configuration for adding renovate. It's like dependabot but it's far less noisy, allowing to update dependencies when ready instead of whenever dependabot pokes you to do so.

The main benefits over dependabot:

• group dependencies. Can even group across different systems, e.g. github-action + pre-commit
• reuse of PR/issues
• flexibility and configurability. This is just a basic configuration file, there are much more that can be configured here.

For an example, you can check my fork where I've enabled this: https://github.com/LecrisUT/tmt/issues/2

To add this the app needs to be installed, and when configuring the settings (403 page because it points to the not-yet configured url), make sure to set it to Renovate -> Mode -> Interactive, otherwise PRs are not created.