customTag = (tagName, onmount, ondestroy) => GLOBALS.CUSTOM_TAGS[tagName] = {
  name: tagName,
  onmount: onmount || (() => {}),
  ondestroy: ondestroy || (() => {}),
  saved: null,
}
@param {string} tagName
@param {function} onmount
@param {function} ondestroy
@returns {object}