Improve match (regex) condition

Great idea, @amaisano

For some reason, `matches` doesn't have the `property` configuration key that many condition plugins have (like `equals`, `greater_than`, etc.). I think the goal of this issue should be to add that same functionality to `matches`.

For BC reasons, I don't think we can change the "parens" behavior in the module. However, once `property` is supported within `matches`, a hook_plugin_info_alter should allow you to change the `parens` setting in the `matches` annotation to be `property`.

There's also some validation we'd have to change. And the `regex` key would no longer be required.

I can't recall exactly why I didn't make the `matches` plugin extend the SimpleComparisonBase class. Maybe that would have been better.

Anyway, we can definitely expand configuration options here but have to be careful to maintain BC.

Found a quick, custom plugin based solution thanks to author's help:

namespace Drupal\sw_migrate\Plugin\migrate_conditions\condition;

use Drupal\migrate\MigrateException;
use Drupal\migrate_conditions\Plugin\SimpleComparisonBase;

/**
 * Alternate to migrate_conditions matches condition.
 *
 * Allows dynamic regex to be passed into regex() operator
 * via source and/or _pseudo_field properties.
 *
 * @MigrateConditionsConditionPlugin(
 *   id = "regex",
 *   parens = "property"
 * )
 */
class Regex extends SimpleComparisonBase {

  /**
   * {@inheritdoc}
   */
  protected function compare($source, $value) {
    if (is_string($source) && !is_null($value)) {
      $regex = '/' . $value . '/';
      return (bool) preg_match($regex, $source);
    }
    else {
      throw new MigrateException('When using the regex condition, the source must be a string.');
    }
  }

}