Make trailing comma syntax errors more clear

[dsnet]

v := jsontext.Value(`["foo", "bar", "baz", ]`)
fmt.Println(v.Compact())

prints:

jsontext: invalid character ']' at start of value within "/3" after offset 22

We should instead report something like:

jsontext: invalid trailing ',' before ']' after offset 20

An accidental trailing comma is a common mistake in hand-modified JSON.
While the current syntactic error is correct (and actually the easiest to produce given the parsing context), it is not particularly easy for a human to diagnose the problem.

\cc @scottjab