composer(deps-dev): update psalm/plugin-phpunit requirement from ^0.16.1 to ^0.18.4

[dependabot[bot]]

Updates the requirements on psalm/plugin-phpunit to permit the latest version.

Release notes

Sourced from psalm/plugin-phpunit's releases.

0.18.4

What's Changed

• Allow Psalm 5.0 by @​ddeboer in psalm/psalm-plugin-phpunit#127

New Contributors

• @​ddeboer made their first contribution in psalm/psalm-plugin-phpunit#127

Full Changelog: https://github.com/psalm/psalm-plugin-phpunit/compare/0.18.3...0.18.4

Commits

• e4ab309 Merge pull request #127 from ddeboer/psalm-5
• df68e66 Bump Psalm to 4.7.1
• e2916b7 Update tests for Psalm 5
• a530027 Suppress BC issues
• 3b306f2 Allow plugin
• 0fac00d Move config to the bottom of the file
• a96d1df Allow Psalm 5.0
• 057c1cd Merge pull request #126 from Nicelocal/master
• eb69695 Fix typo, it turns out $_ is used later on as a ref parameter
• b7aabc4 Merge pull request #125 from Nicelocal/master
• Additional commits viewable in compare view

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