feat: add support for MATCHED_FILES and PACKAGE_FILES step argument

[LMaxence]

Description

Add support for command interpolation of some common arguments, like the list of changed files, and the list of changed files in a given package.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation