Expose input PNG comments in metadata

[nkeynes]

Read-side implementation of PNG text following the proposed API in #3549, ie

comments: [
  { keyword: 'parameters', text: 'highly detailed...' },
  { keyword: 'Copyright', text: 'legal minefield' },
  ...
]

Notes:

• Returns the comments in the order that they're stored in the VipsImage metadata, rather than sorting on the embedded index. It looks like this should give the same ordering under normal circumstances.
• Reuses the existing test fixture testJoinChannel.png that happens to already have a comment, in order to keep change size down.

[lovell]

Hello, thank you very much for this PR, I've left a couple of questions inline.

The failing CI jobs are unrelated to this change. It looks like the latest version of node-gyp no longer supports EOL versions of Python, and the failing environments are all using either 3.6 or 3.7 (latest is 3.12). I'll investigate this separately.

[lovell]

Thank you for the updates, please can you rebase against the latest commit on main to pick up the CI fixes.

[hugh-bita]

look good to me ?

[lovell]

👍