Add modifier for strict schema validation

[sbarfurth]

Description

This adds a modifier strict that can be used with the schema rule. All it does it ensure that a validated schema has no additional properties. This makes a strict schema validation more locked down than comparable coding paradigms like interfaces.

Thank you @dbismut for the idea.

Part of #191, inspired by #179

Type of change

• 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)
• Other (please, feel free to specify what kind of change it is)

Checklist:

• I have created my branch from a recent version of master
• The code is clean and easy to understand
• I have written tests to guarantee that everything is working properly
• I have made corresponding changes to the documentation
• I have added changes to the Unreleased section of the CHANGELOG

[sbarfurth]

For you to note @imbrn: Every modifier function can now also use the rule it is being validated on. The rule is passed to the modifier as a second argument. This change is backward compatible. This allows modifiers to act based on the rule they are on and even to validate based on the arguments given to the rule. This is important for this modifier, because it needs to change the outcome of the validation based on the configuration of the schema rule.

[imbrn]

Thanks @barfurth. I'll take a look at that later.

[imbrn]

It seems really good to me.

I'd like to have some other tests using other modifiers together, for example:

.not.strict
.not.some.strict
.some.not.strict
// etc

It'd be nice to also have some test to check that strict is actually ignored by not-schema-based validations.

[sbarfurth]

@imbrn I have added more tests - what do you think?

[imbrn]

Really nice! 🤩