Previously it was !{, but in real examples this looked confusing since
an exclamation point after a word looks (to humans) like literal
punctuation rather than syntax.
% is not ideal either since it's also the marker traditionally used for
printf, but has the advantage that programmers are already primed for it
to be syntax.
The ~ character can be used at the start and end of interpolation
sequences to trim off whitespace in neighboring literals, with the goal
of allowing extra whitespace to be included for readability without
including it in the resulting string.
