Add SQDBG to this repo

[neochief]

Nearly 6 months ago I've improved existing SQDBG code so that it can work both on Windows and OSX & Linux. I did it just to be able to implement debugger support in the Squirrel plugin for IntelliJ IDEA, but since then I've switched to other stuff.

Sorry, that I'm not posting this as PR, but here's a repo, where you can take its sources: https://github.com/neochief/squirrel-lang-sdk

I would love to make a PR, but I'm not sure how to properly cook the make and cmake files. So, if anyone, including Alberto wants to include a working cross-platform version of SQDBG, be my guest.