Allow passing variables object to mutation.execute()

[bennypowers]

Instead of requiring me to modify element state, allow me to pass variables object directly to the execute (or mutate)? method

// instead of this
mutate(event) {
  const { id } = event.target.dataset
  this.$.mutation.variables = { id }; // modifying state
  this.$.mutation.execute(); // no params
}

// allow this:
newMutate(event) {
  const { id } = event.target.dataset
  this.$.mutation.mutate({ id }); // no params
}