Allow trailing comma in use-names-list

[eamonburns]

Fixes #713

Allow trailing comma in use-names-list to be consistent with other constructs (e.g. include-names-list, record-fields)

use types.{
  foo,
  bar as baz,
  quux,
};

This was an easy-to-miss sidenote, and was essentially an exception to the grammar as written. If trailing commas are meant to be explicitly forbidden, it could more easily be written as one of the following:

use-names-list ::= use-names-item
                 | use-names-item ',' use-names-list

use-names-list ::= use-names-item (',' use-names-item)*

Related: #586

[eamonburns]

It doesn't look like I can add people as reviewers, but @lukewagner mentioned being willing to review.

[lukewagner]

Thanks! Checking in wasm-tools just now, it matches the EBNF, so I guess it is just deleting the misleading line.