Mark public modifier on interface methods as unnecessary

[tmat]

Version Used:
Version 16.8.0 Preview 3.0 [30427.284.main]

Steps to Reproduce:

interface I
{
    public void F();
}

Expected Behavior:

public is marked as unnecessary.

Actual Behavior:

[CyrusNajmabadi]

we acutally have a setting for this: dotnet_style_require_accessibility_modifiers:for_non_interface_members.

We created the setting with the knowledge that DIM was coming, but i don't think it was ever hooked up to work. We should df impl it. If the value is not set to 'always' we should def also fade out.