fix: allow a comment between a trailing comma and ']' (#1500) (#1696)

Trailing commas and comments are both allowed by default, but they did
not compose inside arrays: readArray detected a trailing-comma ']' with a
raw `*current_ == ']'` peek that skipped only whitespace, not comments.
So `[1, 2, /* c */]` left current_ at the comment, the peek failed, and
the parser tried to read another value -- which hit ']' and reported
"value, object or array expected". readObject already handled this via
readTokenSkippingComments.

Add skipCommentTokens() (skip whitespace and comments, leaving current_
at the next significant character) and use it in readArray before the
']' check. Consumed comments stay in commentsBefore_, so a comment before
a real element is still attached to it; if the array ends, they are
simply not attached -- matching object behavior.

Adds CharReaderTest/parseTrailingCommaWithComment covering line/block
comments after a trailing comma, an empty array containing only a
comment, the object form, and that a comment before a real element is
still attached.