[Enhancement] Register I18N-target attributes for custom elements without I18nBehavior

[t2ym]

Register I18N-target attributes for custom elements without I18nBehavior.

Example I18N-target attributes in a static template in the light DOM of i18n-attr-repo:

    <i18n-attr-repo>
      <template id="custom">
        <shop-md-decorator error-message="$"></shop-md-decorator>
      </template>
    </i18n-attr-repo>