Added debugger category with top debugger projects

[TedTran2019]

bgvo talked about a missing category for byebug in issue #859.

I only included debuggers, excluding comprehensive developer toolsets with debuggers included.

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[colszowka]

@TedTran2019 Thank you!

We do already have a similar category "IRB Alternatives" though - https://www.ruby-toolbox.com/categories/irb_Alternatives

I think your choice of name is much more adequate at this point, so maybe let's merge the original category entries with your new "Debuggers" category, and also add irb itself now that it's also a gem?