Update using directive code fix to support blank lines between groups

[sharwell]

As mentioned by @vweijsters during code review in #1785:

We [should] also add a setting that will insert blank lines between the top-level groups, as described in CoreFx coding guidelines point 6.

Marking as needs discussion since this will require a new configuration property which we haven't named yet.

Edit: Rule SA1516 would be updated to report violations according to this setting. The supported values would be allow (default), require, and omit, just like we implemented for a similar situation in #1820.

[sharwell]

I'm marking this as a proposal based on my edit in the original comment.

My vote is 👍

@vweijsters I deleted your 👍 comment since the meaning of this issue changed.

[rkeithhill]

Yes, please make this change to better support the CoreFx coding guidelines.

[vweijsters]

👍 (including the edit)

[vweijsters]

Grabbing this