Added new snippets.

[Dennis (Dennis-Petrov)]

Snippets use string.IsNullOrWhiteSpace() instead of
string.IsNullOrEmpty, as proposed in #190.

[Sam Harwell (sharwell)]

💡 If you add new snippets, you should add their shortcuts to shortcuts.txt (in the same folder). They will also need to be added to the CSharpSnippets in Microsoft.Research/ManagedContract.Setup/ManagedContracts.wxs. If you aren't comfortable with the last one, just open up a new issue and we can address it separately if this gets merged. 😄

📝 This pull request does not supersede (or conflict with) #193.

💭 I am leaning slightly towards #193 (which changes String to string but still uses IsNullOrEmpty). I find that most cases in code which use IsNullOrWhiteSpace incur a performance cost but gain very little in terms of argument validation. I'll wait for feedback from others though.

[Sam Harwell (sharwell)]

💡 You could change this to your name instead. 😄

[Dennis (Dennis-Petrov)]

Thanks, I'll fix shorcuts.txt/WiX script a little later. Concerning performance, these snippets, of course, are not replacement for IsNullOrEmpty family, and should be used only when white space strings are not allowed.