fix: preserve blank line before trailing comments at end of object/array (#34)

[todor-a]

Fixes #34.

Problem

A blank line before a comment at the end of an object (or array) is dropped, even though dprint preserves blank lines before comments everywhere else:

{
  // This is a setting.
  "foo": "",

  // These settings are temporarily commented out.
  // "baz": "",
  // "qux": ""
}

The blank line before // These settings... is removed.

Cause

Comments at the end of a container are emitted via the close-token leading-comments path in gen_surrounded_by_tokens, which calls gen_comments_as_statements with last_node = None. With no previous node, the blank-line gap between the last member and the comment is never computed, so the blank line is lost. Comments between members go through gen_comments_as_leading, which is why they keep their blank line.

Fix

Before emitting the close-token leading comments, add a single newline when the source had a blank line between the preceding token and the first comment. The separating newline is already emitted by the surrounding structure, so only the blank line itself is added (and multiple blank lines still collapse to one).

Applies to both objects and arrays.

Tests

Added tests/specs/comments/Comments_BlankLineBeforeTrailing.txt covering: object (the issue repro), array, the no-blank-line case (no regression), and multiple-blank-line collapse. All specs pass and remain idempotent under format_twice.

[dsherret]

Thanks!