add editorconfig

[OmgImAlexis]

Taken from https://github.com/jquery/api.qunitjs.com/blob/master/.editorconfig as per #293 (comment) suggested.

[scottgonzalez]

The only .json file that uses spaces is package.json. Every other file uses tabs.

[scottgonzalez]

When adding a file like this, you should attempt to apply the rules to the existing codebase. That will give you an idea if the rules you're putting in place actually make sense for the project.

[scottgonzalez]

Why did you close all of your PRs?

[OmgImAlexis]

@scottgonzalez I'll just wait till you guys have your repos worked out. There's no point in keeping them open when I can't even merge them as you guys haven't added an editorconfig or anything similar to keep whitespace setup correctly. I would have figured from an organisation as big as jQuery these things would have been worked out years ago.