function extensionBase() {
    return {
      init: function(api) { return null },
      getSelectors: function() { return null },
      onEvent: function(name, evt) { return true },
      transformResponse: function(text, xhr, elt) { return text },
      isInlineSwap: function(swapStyle) { return false },
      handleSwap: function(swapStyle, target, fragment, settleInfo) { return false },
      encodeParameters: function(xhr, parameters, elt) { return null }
    }
  }
extensionBase defines the default functions for all extensions.
@returns {HtmxExtension}