chore: make engine pluggable

[didroe]

Description

Makes the engine pluggable. For now, we need to also make all the packages public.

Also:

• Moves serializable regex type to the regex package
• Refactors settings to avoid an import cycle

Checklist

If this is your first time contributing please sign the CLA

• I've added test coverage that shows my fix or feature works as expected.
• I've updated or added documentation if required.