Better unknown handling configuration

[imjoshin]

Fixes #2739

Status: Draft
This is still a draft as I haven't been able to dedicate the time required to fix up the tests. I'm open to someone else helping out and/or providing guidance. 😄

This PR aims to have better handling of unknown keys. Within the validate configuration, instead of just a boolean, allowUnknown supports clearer definitions with backwards compatibility:

type AllowMode = boolean | 'allowed' | 'forbidden' | 'warn';
...

allowUnknown?: AllowMode | {
    mode: AllowMode,
    type?: string,
};

These means we can either allow unknown keys with allowed or true, reject with forbidden or false, or warn with warn. If warn or forbidden are set, we can set what type the warning or error object are by setting the type property.

[imjoshin]

We'll definitely want more tests here to cover root level unknown as well as allowUnknown: 'forbidden'