Implement UseOnlySpaces

[mdiasdev]

No description provided.

[abl]

Thanks for taking this on! Of note, we weren't going to use SwiftSyntax to implement this rule - the use of whitespace isn't dependent on syntax.

Arguably, we might want to avoid changing whitespace characters inside of strings silently - that could create some surprising behavior when folks copy/paste. The tab-to-eight-spaces feels safe, though.

@allevato thoughts?