[8.x] Make Validator::parseNamedParameters public

[ragulka]

This change will allow custom validation rules having access to the validator to parse named parameters.

Consider a custom validation rule using a closure or extension:

Validator::extend('custom_rule', function($attribute, $value, $parameters, $validator) {
  // the parameters are something like ['country=US', 'level=pro'],
  // but we want ['country' => 'US', 'level' => 'pro'] instead
  dd($parameters);
  // this will give us what we want (but currently it fails, because the method is protected)
  dd($validator->parseNamedParameters($parameters));
});

Validator::make([
  'some_field' => 'custom_rule:country=US,level=pro'
], $data);

There are no breaking changes, it simply exposes a helpful method to custom validation rules.

[driesvints]

Just want to note that this is a breaking change.