Add Directory.Build.props

[iPazooki]

Add Directory.Build.props

This pull request resolves issue #345 and adds Directory.Build.props to the solution at the solution level to avoid code duplication.

Checklist before requesting a review

• [x] I have performed a self-review of my code.
• [x] I have built and run all test cases successfully.

Changes

• Add Directory.Build.props files
• Add its reference to the solution file

Notes

• It's possible to encounter null exceptions in many places, so adding <Nullable>enable</Nullable> to the Directory.Build.props file could help prevent them. However, updating the affected projects would be necessary.

• Issue #345