Add Nullable reference type annotations

[udaken]

This PR has three parts.
The behavior is unchanged throughout.

Applying the Roslyn style code format.

• ae4e3bd

The source code styles were not consistent.
We have added Roslyn Style .editorconfig that has many matching styles.

Modernization.

• 768de4d
• 5ad112a
• 585579a

Addition of NRT annotations.(fix #109 )

• 44824be

Nullabe's policy is as follows

• All payloads are Nullabe.
• Nullable if the argument is directly specified as null internally, such as an optional argument.
• Follow the annotations in .NET6.

Unfortunately, if Nullable is enabled, more than 100 warnings will remain.

[xoofx]

Thanks!