Microsoft.CodeAnalysis.PublicApiAnalyzers RS0016 code fix should respect project's NRT setting.

**Version Used**: Microsoft.CodeAnalysis.PublicApiAnalyzers v4.14.0

**Steps to Reproduce**:

1. Add the analyzer to a new project
2. Apply the code fix

**Diagnostic Id**:

RS0016

**Expected Behavior**:

The code fix for RS0016 should add `#nullable enable` to newly created PublicAPI.*.txt files.

**Actual Behavior**:

If there is no existing PublicAPI.*.txt file, the code fix for RS0016 creates a new one which does not include `#nullable enable`, causing RS0037 warnings. Adding `#nullable enable` manually afterwards will give RS0036 warnings because the code fix for RS0016 did not include nullable annotations.

Locate the RS0016 code-fix implementation and the logic that creates new PublicAPI.*.txt files. Check how the project's nullable reference types setting is determined, then verify that a newly created file handles nullable annotations consistently and no longer produces the reported RS0037 or RS0036 warnings.

Written by the indexing model from the issue text.