It is assumed that DOM elements or related objects will have been
escaped before they are passed back from templating functions. As
strings are typically blinding concatenated, like in our defaults,
it makes sense to escape the markup within them.
This is related to https://github.com/select2/select2/issues/3005.
This is needed to escape any bad markup that is passed through
user-entered data. Users can prevent their markup from being
escaped by using a no-op `escapeMarkup` function.
This closes https://github.com/select2/select2/issues/2990.