cli: add format flag to support PNG output to stdout

[Maricaya]

Add --format flag to enable PNG output to stdout, allowing users to pipe D2 diagram output in PNG format to other tools.

Changes:

• Add format flag support for PNG output to stdout
• Add test cases for format flag handling
• Update getOutputFormat to handle format flags properly

Example usage:

d2 input.d2 --stdout-format png -

Test: ./make.sh test

[Maricaya]

closes #2214

[Maricaya]

Hi @alixander this is my first PR for D2, please let me know if I’ve missed anything!

[alixander]

Thank you for your first contribution! Looks mostly good, just some minor comments

You'll need to update the man page too please: https://github.com/terrastruct/d2/blob/master/ci/release/template/man/d2.1

and add a line to next.md as a new feature: https://github.com/terrastruct/d2/blob/master/ci/release/changelogs/next.md

[Maricaya]

Hi @alixander, Thank you for your detailed review and guidance! I've completed the requested updates. 😊