Expose `DeclarationModifiers.Fixed` publicly

[DoctorKrolic]

Background and Motivation

DeclarationModifiers is publicly available for code generation related workflows and contains all possible declaration modifiers. Previously it was missing fixed modifier as it is quite rare in the wild. However, after a bug discovery it was added as an internal API in #78698. I don't see much reason in keeping it internal.

Proposed API

namespace Microsoft.CodeAnalysis.Editing;

public readonly record struct DeclarationModifiers
{
+    public bool IsFixed { get; }
+    public DeclarationModifiers WithIsFixed(bool isFixed)

+    public static DeclarationModifiers Fixed { get; }
}

Usage Examples

#78698

Alternative Designs

Risks

None I can think of