improve linq query support

[louietan]

Hi, thanks for this awesome library.

This PR doesn't introduce any new features, just makes existing linq query syntax support complete.

I accidently found the linq query syntax usage from #566.
But the support is incomplete, so I just added the missing implementations (Task/ValueTask/SelectMany etc.),
along with code examples and a brief introduction in the README, so people know this alternative usage exists.

Motivation

Lots of functional languages actually have this syntax sugar, like do-notation in Haskell, for-comprehension in Scala and computation expressions in F#.

C# is not a functional language, but the idea behind the linq syntax is the same, they provide a more concise syntax for the composition of Maybe/Result types without nested function composition or method chaining.

Instead of:

var customer = Email
 	.Create("[email protected]")
	.Bind(email =>
		CustomerName.Create("jsmith").Map(name => new Customer(name, email))
	);

We can use linq query to flatten the code:

var customer =
	from email in Email.Create("[email protected]")
	from name in CustomerName.Create("jsmith")
	select new Customer(name, email);