xunit: define codefixes for SpanAsserts

The asserts defined in https://github.com/xunit/assert.xunit/blob/main/SpanAsserts.cs

- [ ] `Contains(Span expectedSubSpan, Span actualSpan)`
- [ ] `Contains(Span expectedSubSpan, ReadOnlySpan actualSpan)`
- [ ] `Contains(ReadOnlySpan expectedSubSpan, Span actualSpan)`
- [ ] `Contains(ReadOnlySpan expectedSubSpan, ReadOnlySpan actualSpan)`
- [ ] `Contains(Span expectedSubSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `Contains(Span expectedSubSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `Contains(ReadOnlySpan expectedSubSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `Contains(ReadOnlySpan expectedSubSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `Contains(Span expectedSubSpan, Span actualSpan)`
- [ ] `Contains(Span expectedSubSpan, ReadOnlySpan actualSpan)`
- [ ] `Contains(ReadOnlySpan expectedSubSpan, Span actualSpan)`
- [ ] `Contains(ReadOnlySpan expectedSubSpan, ReadOnlySpan actualSpan)`
---
- [ ] `DoesNotContain(Span expectedSubSpan, Span actualSpan)`
- [ ] `DoesNotContain(Span expectedSubSpan, ReadOnlySpan actualSpan)`
- [ ] `DoesNotContain(ReadOnlySpan expectedSubSpan, Span actualSpan)`
- [ ] `DoesNotContain(ReadOnlySpan expectedSubSpan, ReadOnlySpan actualSpan)`
- [ ] `DoesNotContain(Span expectedSubSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `DoesNotContain(Span expectedSubSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `DoesNotContain(ReadOnlySpan expectedSubSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `DoesNotContain(ReadOnlySpan expectedSubSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `DoesNotContain(Span expectedSubSpan, Span actualSpan)`
- [ ] `DoesNotContain(Span expectedSubSpan, ReadOnlySpan actualSpan)`
- [ ] `DoesNotContain(ReadOnlySpan expectedSubSpan, Span actualSpan)`
- [ ] `DoesNotContain(ReadOnlySpan expectedSubSpan, ReadOnlySpan actualSpan)`
---
- [ ] `EndsWith(Span expectedEndSpan, Span actualSpan)`
- [ ] `EndsWith(Span expectedEndSpan, ReadOnlySpan actualSpan)`
- [ ] `EndsWith(ReadOnlySpan expectedEndSpan, Span actualSpan)`
- [ ] `EndsWith(ReadOnlySpan expectedEndSpan, ReadOnlySpan actualSpan)`
- [ ] `EndsWith(Span expectedEndSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `EndsWith(Span expectedEndSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `EndsWith(ReadOnlySpan expectedEndSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `EndsWith(ReadOnlySpan expectedEndSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
---
- [ ] `Equal(Span expectedSpan, Span actualSpan)`
- [ ] `Equal(Span expectedSpan, ReadOnlySpan actualSpan)`
- [ ] `Equal(ReadOnlySpan expectedSpan, Span actualSpan)`
- [ ] `Equal(ReadOnlySpan expectedSpan, ReadOnlySpan actualSpan)`
- [ ] `Equal(Span expectedSpan, Span actualSpan, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false, bool ignoreAllWhiteSpace = false)`
- [ ] `Equal(Span expectedSpan, ReadOnlySpan actualSpan, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false, bool ignoreAllWhiteSpace = false)`
- [ ] `Equal(ReadOnlySpan expectedSpan, Span actualSpan, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false, bool ignoreAllWhiteSpace = false)`
- [ ] `Equal(ReadOnlySpan expectedSpan, ReadOnlySpan actualSpan, bool ignoreCase = false, bool ignoreLineEndingDifferences = false, bool ignoreWhiteSpaceDifferences = false, bool ignoreAllWhiteSpace = false)`
- [ ] `Equal(Span expectedSpan, Span actualSpan)`
- [ ] `Equal(Span expectedSpan, ReadOnlySpan actualSpan)`
- [ ] `Equal(ReadOnlySpan expectedSpan, Span actualSpan)`
- [ ] `Equal(ReadOnlySpan expectedSpan, ReadOnlySpan actualSpan)`
---
- [ ] `StartsWith(Span expectedStartSpan, Span actualSpan)`
- [ ] `StartsWith(Span expectedStartSpan, ReadOnlySpan actualSpan)`
- [ ] `StartsWith(ReadOnlySpan expectedStartSpan, Span actualSpan)`
- [ ] `StartsWith(ReadOnlySpan expectedStartSpan, ReadOnlySpan actualSpan)`
- [ ] `StartsWith(Span expectedStartSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `StartsWith(Span expectedStartSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `StartsWith(ReadOnlySpan expectedStartSpan, Span actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`
- [ ] `StartsWith(ReadOnlySpan expectedStartSpan, ReadOnlySpan actualSpan, StringComparison comparisonType = StringComparison.CurrentCulture)`