Restrict prop names and slot names to allowed twig/php variable names

Christian López Espínolarequested to merge
issue/drupal-3465228:3465228-twig-disallows-dashes into main
  • Restrict prop names and slot names to allowed twig/php variable names: starting with letter or underscore, can contain digits, letter, underscore.
  • Enabled schema validation for prop names too.

Closes #3465228

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