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. 😊

[alixander]

@Maricaya this looks good to me, thanks for addressing the comments.

I'm not sure what happened but only one of your commits is signed, would you mind fixing that? Ready to merge after

[alixander]

#2291