function getInternalData(elt) {
    const dataProp = 'htmx-internal-data'
    let data = elt[dataProp]
    if (!data) {
      data = elt[dataProp] = {}
    }
    return data
  }
getInternalData retrieves "private" data stored by htmx within an element
@param {EventTarget|Event} elt
@returns {HtmxNodeInternalData}