Disallow illlegal trailing commas in order by clause

[tomharmon]

Currently on master:

nom-sql/src/order.rs

Line 58 in 026473b

order_expr: many0!(

In src/order.rs, the order by clause allows for trailing commas, which is technically invalid SQL.
Eg, "select * from users order by name ascage desc,\n" will parse, but should not

In the incoming bump nom to v5 PR, this is on line 76 of the same file:

76        many0(order_expr),