added leetcode 78 golang backtracking subsets

[GokhanCagritekin]

No description provided.

[mitchellirvin]

hi @GokhanCagritekin ! thank you for contributing!

a couple small things:

1. this is currently admittedly inconsistent, and we do need to figure out a better long term approach, but do you mind moving all of your solutions to the go/ directory (instead of inside backtracking/)? in the future we'll get a neetcode_150 directory set up with all of the categories, but until then let's keep this at the top level
2. do you mind updating your file names to match the convention that exists elsewhere in the repo? currently it's <problem-number>-Name-of-Problem.<language-extension> see the contributing guidelines for details

[GokhanCagritekin]

Hi @mitchellirvin . Thank you for your feedback. I just updated my pull request.

[Ahmad-A0]

Thanks, @GokhanCagritekin!