Add assertions on JsonElement

[vbreuss]

Draft for #2559

Add a new class JsonElementAssertions which contains assertion methods on JsonElement?).
This class can then be extended with assertions similar to JTokenAssertions in FluentAssertions.Json.

In the first step, I would keep the public surface quite small and only add the following methods:

/// <summary>
/// Asserts that the number of items in the current <see cref="JsonElement" /> matches the supplied <paramref name="expected" /> amount.
/// </summary>
public AndConstraint<TAssertions> HaveCount(int expected, string because = "", params object[] becauseArgs);

/// <summary>
/// Asserts that the number of items in the current <see cref="JsonElement" /> does not match the supplied <paramref name="unexpected" /> amount.
/// </summary>
public AndConstraint<TAssertions> NotHaveCount(int unexpected, string because = "", params object[] becauseArgs);

/// <summary>
/// Asserts that the current <see cref="JsonElement" /> has a direct child element with the specified <paramref name="expected" /> name.
/// </summary>
public AndWhichConstraint<TAssertions, JsonElement> HaveElement(string expected, string because = "", params object[] becauseArgs)

/// <summary>
/// Asserts that the current <see cref="JsonElement" /> has no direct child element with the specified <paramref name="expected" /> name.
/// </summary>
public AndConstraint<TAssertions> NotHaveElement(string expected, string because = "", params object[] becauseArgs)

In order to use these assertions, I would add the following methods to the AssertionExtensions:

public static JsonElementAssertions Should(this JsonDocument actualValue);
public static JsonElementAssertions Should(this JsonElement? actualValue);

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR adds a feature or fixes a bug, please update the release notes with a functional description that explains what the change means to consumers of this library, which are published on the website.
• If the PR changes the public API the changes needs to be included by running AcceptApiChanges.ps1 or AcceptApiChanges.sh.
• If the PR affects the documentation, please include your changes in this pull request so the documentation will appear on the website.
  • Please also run ./build.sh --target spellcheck or .\build.ps1 --target spellcheck before pushing and check the good outcome

[github-actions]

Qodana for .NET

44 new problems were found

Inspection name	Severity	Problems
Auto-property accessor is never used (private accessibility)	🔶 Warning	19
Non-accessed field (private accessibility)	🔶 Warning	13
Possible 'System.NullReferenceException'	🔶 Warning	4
Redundant type declaration body	🔶 Warning	3
Disposal of a variable already captured by the 'using' statement	🔶 Warning	2
Member hides static member from outer class	🔶 Warning	1
Return value of a method annotated with [MustDisposeResource] is never disposed	🔶 Warning	1
Possible 'System.InvalidOperationException'	🔶 Warning	1

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[coveralls]

Pull Request Test Coverage Report for Build 7520186901

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 97.459%

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Totals
Change from base Build 7517844575:	-0.02%
Covered Lines:	11807
Relevant Lines:	11991

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💛 - Coveralls