Methods like `parse_custom_script_rule` should be refactored to `Pairs`

[wirednkod]

This is a follow up to @l0r1s 's comment

[pepoviola]

@wirednkod / @l0r1s is this still needed right? I think is an small task that we can complete at some point when we add some other rules to the parser. Thx!

[l0r1s]

@pepoviola Refactor is always needed ! It's not a priority but could make things cleaner for sure