Use syntax-span and syntax-position to compute the tail space in a list

[sorawee]

@racketblock[( a   b   cde  )]

is currently rendered as:

( a   b   cde)

The spaces between "cde" and ")" are missing.

One possible fix:

• use syntax-position to find that the position of "( a .... )" is x with the span of 16
• use syntax-position to find that the position of "cde" is x + 10 with the span of 3
• So there should be 16 - ((x + 10) - x) - 3 - 1 = 2 spaces between "cde" and ")".
• More generally: the quantity is whole-span - (last-pos - whole-pos) - last-span - 1.
• The spaces must be added before ")" is written. Somewhere before https://github.com/racket/scribble/blob/bdffb99ca49ed17be8198f2376e6bf6b199cb813/scribble-lib/scribble/racket.rkt#L788