[core] Support environment variable CLASSPATH with pmd.bat under Windows

[adangel]

Describe the PR

Fixes the problem found in #4391

The environment CLASSPATH is supported in run.sh for a long time, but this was obviously never added to pmd.bat.

Note: This needs to be ported to PMD 7 as well - there we seem to support CLASSPATH only in the bash script, but not in pmd.bat.

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)

[pmd-test]

	1 Message
📖	No regression tested rules have been changed.

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