add excimer extension

[henderkes]

What does this PR do?

closes #1017

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option.
If a modification is not involved, please skip it directly.

• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • composer cs-fix
  • composer analyse
  • composer test
  • bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • Add your test combination to src/globals/test-extensions.php.
  • If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.

[henderkes]

4 more weeks until the macos 8.5 bug is finally fixed

[crazywhalecc]

Code part looks good, but I haven't tested locally yet.

[henderkes]

Feel free to merge when you tested. I didn't care to test the extension past a php -i | grep excimer.