Create Analyzer/Fixer for Assert.Single

[RieBi]

This is intended to solve the final part of xunit/xunit#1510 (if I haven't missed anything).

It maps the single-argument call to Assert.Single with linq where expression, and suggests to change it to two-argument call:

- Assert.Single(list.Where(f => f > 0));
+ Assert.Single(list, f => f > 0);

[RieBi]

I would love to receive feedback if there's anything wrong @bradwilson

[bradwilson]

Sorry, I will get to this but I haven't had time yet. My apologies.

[bradwilson]

Thanks, everything looks great! I made a couple minor edits.