Migration process plugin for SmartTrim logic

The new trait is a neat way of providing the trim functionality for both the formatter and the process plugin. However, it was missing a use statement in the classes to actually implement the trait. I renamed it to SmartTrimTrait, as SmartTrim was conflicting with the name of the SmartTrim plugin.

Should the input and output of the trim() method be typed? I considered giving it a string type, but I wasn't certain it would always be a string.

Does it need a test of the migration?

Also note currently PHPstan (next minor) is failing. The failure is in the TruncateHTML class, which hasn't been changed by this issue, so I think it's caused by an upstream change, so I think it should be looked at by a separate issue.

We also might be able to replace the TruncateHTML file with the trait... That file probably needs a lot of clean out anyway.

Thanks for the fixes, lostcarpark!