Show diagnostics on abandoned cancellation token callback registration

[ycherkes]

Abandoned cancellation token callback registration causes memory leaks.

Analyzer generates following diagnosticts for code:

// IDISP001 | Dispose created
using var cts = new CancellationTokenSource();
var token = cts.Token;
↓CancellationTokenRegistration ctr = token.Register(() => client.CancelAsync());

// IDISP004 | Don't ignore created IDisposable
using var cts = new CancellationTokenSource();
var token = cts.Token;
↓token.Register(() => client.CancelAsync());

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