[Code Style] Add some extra style checks focused on nullability.

[mandel-macaque]

Added the following new checks:

// dotnet_style_coalesce_expression = true
var v = x ?? y;

// dotnet_style_coalesce_expression = false
var v = x != null ? x : y; // or
var v = x == null ? y : x;

// dotnet_style_null_propagation = true
var v = o?.ToString();

// dotnet_style_null_propagation = false
var v = o == null ? null : o.ToString(); // or
var v = o != null ? o.String() : null;

// dotnet_style_prefer_is_null_check_over_reference_equality_method = true
if (value is null)
    return;

// dotnet_style_prefer_is_null_check_over_reference_equality_method = false
if (object.ReferenceEquals(value, null))
    return;

// dotnet_style_prefer_is_null_check_over_reference_equality_method = false
if ((object)o == null)
    return;