panc: trailing comma in choice type causes parse error

[jrha]

Other types and functions handle a trailing comma gracefully.

This pollutes diffs when long lists of choices are written out one-per-line.

bind '/g' = choice(
    'a very long option',
    'another slightly longer option',
);

Results in:

Encountered: )
Was expecting one of:
    <HEREDOC_STRING> ...
    <SINGLE_QUOTED_STRING> ...
    <DOUBLE_QUOTED_STRING> ...