fix: accept SELECT-list trailing commas at query boundaries

[apstndb]

Fixes #404

Spanner accepts trailing commas in shared SELECT lists at subquery and pipe boundaries. This also matches the optional trailing comma in GoogleSQL's shared select_list grammar.

Summary

• accept SELECT-list trailing commas before ) and |>
• cover parenthesized, EXISTS, and ARRAY subqueries as well as pipe SELECT
• preserve canonical SQL output without the trailing comma

[makenowjust]

👍