Add accessibility modifiers when absent

[cybertyche]

This PR ensures that all classes, interfaces, class members, etc. have accessibility modifiers present. This was the result of a tool run by Visual Studio followed by a lot of manual cleanup (for some reason, the tool decided to remove readonly modifiers, which was odd so I added them back).

In one or two cases only, I changed a private readonly to a private const where it made sense to do so as part of that manual cleanup.

Microsoft Reviewers: Open in CodeFlow