Add BeValidJson extensions for strings

[vbreuss]

Draft for #2556

Add the following methods to the StringAssertions (similar to StringAssertionsExtensions in FluentAssertions.Json):

public AndWhichConstraint<TAssertions, JsonDocument> BeValidJson(string because = "", params object[] becauseArgs);
public AndWhichConstraint<TAssertions, JsonDocument> BeValidJson(JsonDocumentOptions options, string because = "", params object[] becauseArgs);

These methods should assert that the string is a valid JSON document.

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[coveralls]

Pull Request Test Coverage Report for Build 7519123647

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 97.476%

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Totals
Change from base Build 7517844575:	-0.004%
Covered Lines:	11775
Relevant Lines:	11958

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💛 - Coveralls

[ivarne]

This way of using JsonDocument seems to be against the documentation that states that it is important to dispose (or clone) JsonDocument instances. I'm not sure I fully understand the consequences they warn about here, but my suggestion would be to use the raw Utf8JsonReader for BeValidJson and not provide the document for further usage.

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/use-dom#jsondocument-is-idisposable

JsonDocument builds an in-memory view of the data into a pooled buffer. Therefore the JsonDocument type implements IDisposable and needs to be used inside a using block.