[4.2] Convert the checkfiles plugin to service provider

[laoneo]

Summary of Changes

Converts the checkfiles plugin to service provider and adds some unit tests. This pr is based on #38060, so please test that one first.

Testing Instructions

• Create a folder in images with the name test and upload there a big image from unsplash or so which is bigger than 1080 pixels
• Go to the task scheduler back end list
• Click on new "Image Size Check"
• Select the test image in the task parameters
• Save the task and close the form
• Run the task by clicking on "Run Test" button

Actual result BEFORE applying this Pull Request

The image in the test folder has a width or height of 1080 pixels.

Expected result AFTER applying this Pull Request

The image in the test folder has a width or height of 1080 pixels.

[alikon]

I have tested this item ✅ successfully on db97381

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38072.}

[joomla-bot]

A new pull request has been created automatically to convert this PR to the PSR-12 coding standard. The pr can be found at Digital-Peak#24

[HLeithner]

A new pull request has been created automatically to convert this PR to the PSR-12 coding standard. The pr can be found at Digital-Peak#24

there was a merge error on this pull request please check if worked correctly

[laoneo]

I had to redo this pr, @heelc29 can you have a look again if I missed something. Thanks!

[heelc29]

the same changes 😅