Add an objectStreams option to write PDF 1.5 object streams

[tomsotte]

What kind of change does this PR introduce?

A feature, closes #1810. It adds an opt-in objectStreams option that writes PDF 1.5 object streams and a cross-reference stream.

Objects without a stream are packed into compressed /ObjStm objects, up to 200 per stream. Examples are structure elements, annotations, font dictionaries and the catalog. A single /XRef stream replaces the xref table and the trailer. How much smaller a file gets depends on how many of these small objects it has, so tagged documents gain the most:

• A tagged PDF/A-2a document with a large table went from 1.73 MB to 415 KB. For comparison, qpdf --object-streams=generate makes the same file 379 KB.
• pdfkit's examples, which are mostly images and fonts, get 1–27% smaller.

Changes:

• It's off by default, and the default output doesn't change. I checked byte for byte on the examples.
• The logic is in a new lib/object_streams.js:
  • reference.js hands it the objects that have no stream.
  • document.js has it write the xref stream at the end.
  • Output keeps streaming, buffering at most 200 serialized objects.
• _refEnd now resets _ended before calling _finalize. Writing the last object stream ends one more reference, and without this reset the xref would be written twice.
• It throws:
  • with a pdfVersion below 1.5;
  • with PDF/A-1, which forbids object streams;
  • with encryption, which is possible per the spec but left out to keep this PR small;
  • when the file goes past 4 GB, because the xref stream writes offsets in 4 bytes.

Testing:

• Unit tests cover the xref stream, the packed objects, splitting into several object streams, references ended late, compress: false, tagged PDF/A-2, and every case that throws.
• With the option forced on for every document, all visual tests still pass. Every PDF from the test suite then passes qpdf --check, except the ones that already failed it, which have no pages or are encrypted.
• With the option on, the examples render identically in poppler. veraPDF reports the same results with and without the option for PDF/A-2b, PDF/A-3b and PDF/UA.

Checklist:

• Unit Tests
• Documentation
• Update CHANGELOG.md
• Ready to be merged

I wrote this with the help of AI, then reviewed the code and checked the output myself.

[blikblum]

LGTM Please fix the conflict