Feature: add optional and nullable rules

[joshcummingsdesign]

This PR removes the Validator::validateRulesandValues method in favor of new rules. The validation method was deemed unnecessary, as values without rules are already omitted, and rules without values return a null value — the validator itself doesn't need to throw any exceptions as the end result is still safe data.

Example of data omitted without rules:

$request = [
  'firstname' => 'Jason',
  'lastname' => 'Adams',
  'city' => 'Carlsbad',
];

$validator = new Validator([
  'firstname' => ['required'],
  'lastname' => ['required'],
], $request);

$expected = [
  'firstname' => 'Jason',
  'lastname' => 'Adams',
];

When reviewing the following, note that if a field is omitted entirely from the values, then its considered null. So both optional and nullable would work for omitted values.

Optional

If the value is null or an empty string, then the remaining validation rules are skipped.

Example:

$request = [
  'firstname' => 'Jason',
  'lastname' => 'Adams',
  'age' => ''
];

$validator = new Validator([
  'firstname' => ['required'],
  'lastname' => ['required'],
  'age' => ['optional', 'integer']
], $request);

$expected = [
  'firstname' => 'Jason',
  'lastname' => 'Adams',
  'age' => ''
];

Nullable

If the value is null, then the remaining validation rules are skipped.

Example:

$request = [
  'firstname' => 'Jason',
  'lastname' => 'Adams',
  'city' => null
];

$validator = new Validator([
  'firstname' => ['required'],
  'lastname' => ['required'],
  'city' => ['nullable', 'in:san diego'],
], $request);

$expected = [
  'firstname' => 'Jason',
  'lastname' => 'Adams',
  'city' => null,
];

[JasonTheAdams]

@borkweb Since I ended up taking over this PR, I'd appreciate if if you'd give this a code review. Thanks!

[JasonTheAdams]

@joshcummingsdesign needs this for his work, so we went back through this and reviewed it together. It's passing all tests and works, so I'm going to mark this approved, merge, and release it!