xunit: define codefixes for StringAsserts with StringComparison

[Meir017]

Remaining issues from #145

Assertions with StringComparison comparisonType

• [ ] Contains(string expectedSubstring, string actualString, StringComparison comparisonType)
• [ ] DoesNotContain(string expectedSubstring, string actualString, StringComparison comparisonType)
• [ ] Equal(string expected, string actual, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false, bool ignoreAllWhiteSpace = false)
• [ ] EndsWith(string expectedEndString, string actualString, StringComparison comparisonType)
• [ ] StartsWith(string expectedStartString, string actualString, StringComparison comparisonType)