Add the standard phpBB extension tests workflow

[bonelifer]

Summary

• Adds .github/workflows/tests.yml, the standard phpBB extension test workflow (phpbb-extensions/test-framework@3.3.x) that the other phpBB Modders extensions already use. On every push and pull request to the default branch it installs phpBB 3.3 with this extension and runs phpBB's code sniffer, the image and executable-file checks, and EPV (phpBB's extension validator, the same one phpBB.com's validation uses).
• The database, PHPUnit and Windows jobs are off, the same as most of the other extensions, because this extension has no unit tests yet.

Fixes for problems the first CI run found:

• Strips the embedded ICC colour profile from styles/prosilver/theme/images/valentine.png (same 96×96 image otherwise).
• Removes the executable bit from .gitignore, README.md and composer.json.
• Keeps the existing ci.yml (PHP 7.1–8.1 syntax check); the two workflows check different things.

Test plan

• This PR's own CI runs are the first runs of the workflow on this repo. The first run failed on the problems fixed above; the final run passes.

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Investigated and written by Claude on behalf of William Jacoby (bonelifer).