Allow trailing commas in import globs

[erickt]

Pretty much every other construct allows trailing commas, so glob imports like this should work too:

import elasticsearch::{
    client,
    create_index_builder,
    index_builder,
    json_dict_builder,
};

[lkuper]

I'll take this one. Incidentally, 2075622 should make it slightly easier to tell where trailing separators are currently allowed/disallowed.