Allow interpolated strings in .panics

[coke]

For example:

token list {
    \s*
    [
    | <EXPR=.braced_word>
        [ $<extra>=(\S+) <.panic: "list element in braces followed by \"$<extra>\" instead of space" > ]?
    | <EXPR=.list_word>
    ] ** [\s+]
}