let-optionals can be provided by the (chibi optional) library or by
SRFI 227. Either dependency is non-trivial and makes it tricky to
incorporate (chibi sxml) into outside projects. Since (chibi sxml)
only makes trivial use of let-optionals, expand the macro by hand.
Before, it was necessary to do something like this:
(option (@ (selected . #false) (value "any")) "any")
instead of:
(option (@ (selected) (value "any")) "any")
Only the former is valid SXML, as far as I can tell from the SXML
specification:
<https://dl.acm.org/doi/pdf/10.1145/571727.571736>
