[Validator] Added isValid() getter

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no] (maybe the interface?)
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [no]
| License       | MIT

A shortcut for direct validation, with the same API of Form.

```php
// Before:
if($v->validate($entity)->count() === 0)
{
// Do something
}

// After:
if($v->validate($entity)->isValid())
{
// Do something
}