Solves problem with second-level dependencies

[averissimo]

Related to insightsengineering/verdepcheck#38

In scenarios where the DESCRIPTION file has top-level dependencies (Depends, Imports, Suggests, Extends) that depend on packages not yet released on CRAN, this new option allows to define repositories that may fill the gap.

In particular, this is important on max strategy, as they might depend on secondary packages that have changes that only exist on the repository.

Changes description

• Adds new option to add extra repositories
  • If user wants to fully override default repository, they can replace environmental variables
• Adds styling to first (manual) header
• Better organizes readme

TODO:

• Understand how to use this new option in github action matrix

Use case examples

This will be especially helpful with max strategy to allow more packages to be compiled with latest development versions

• insightsengineering packages can use r-universe.dev to pull secondary dependencies such as formatters
• Pull all latest development from tidyverse packages by using https://tidyverse.r-universe.dev/builds

release strategy can also use this if downstream packages are not being release on CRAN

min_*** strategy shouldn't use this option

[averissimo]

See here a workflow run with this PR in tmg's max strategy.

We might be able to do the same using environment variables.