Refactored FluentMapping into separate assembly - #991

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-dotnet@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Non-nullable property 'ExcludeNullableType' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Non-nullable property 'MemberAccessor' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Non-nullable property 'Key' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Possible null reference argument for parameter 'instance' in 'object? MiniExcelMemberAccessor.GetValue(object instance)'.

Argument of type 'List<MiniExcelColumnMapping>' cannot be used for parameter 'mappings' of type 'List<MiniExcelColumnMapping?>' in 'CellWriteInfo[] AsyncEnumerableWriteAdapter<T>.GetRowValues(T currentValue, List<MiniExcelColumnMapping?> mappings)' due to differences in the nullability of reference types.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Argument of type 'List<MiniExcelColumnMapping>' cannot be used for parameter 'mappings' of type 'List<MiniExcelColumnMapping?>' in 'CellWriteInfo[] EnumerableWriteAdapter.GetRowValues(object currentValue, List<MiniExcelColumnMapping?> mappings)' due to differences in the nullability of reference types.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.