Add TrySingle overloads for

[elianora]

Addresses #631 by adding and unit testing the following:

public static TSource? TrySingle<TSource>(this IEnumerable<TSource> source);
public static ValueTask<TSource?> TrySingle<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default)

[viceroypenguin]

One minor nit. Looks great though! Thank you!

[viceroypenguin]

Build errors aren't your fault. I'm gonna fix those real quick and then I'll rebuild your PR.

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 95.28%. Comparing base (f5001c2) to head (28789df).

Files	Patch %	Lines
Source/SuperLinq.Async/TrySingle.cs	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #643   +/-   ##
=======================================
  Coverage   95.28%   95.28%           
=======================================
  Files         248      248           
  Lines        8701     8726   +25     
  Branches     1609     1615    +6     
=======================================
+ Hits         8291     8315   +24     
  Misses        218      218           
- Partials      192      193    +1     

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