devel-docs: add a new file "debugging-tips.txt".
This will be a neat reminder for some of the hidden environment variables helpful to debug GIMP in some cases, as well as some other tricks, which we often have to tell people or even which we forget (or don't even know sometimes) ourselves. This will be less annoying to just remember to check this file if we forget the name of some environment variable than to try and grep the git log or the code. If someone adds another environment variable which changes some behavior of the code, please add it here. Also if you know some other helpful tricks, share with all. Thanks.
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