Preserve interpolation alignment in MA0052 code fix

[meziantou]

Summary

The MA0052 code fix (replace constant Enum.ToString with nameof) dropped the alignment clause of interpolations, which changed the output of the code:

enum Color { Red }

_ = $"[{Color.Red,10}]";          // "[       Red]"
_ = $"[{nameof(Color.Red)}]";     // "[Red]" — previous fix result
_ = $"[{nameof(Color.Red),10}]";  // "[       Red]" — new fix result

Changes

• ReplaceEnumToStringWithNameofFixer now updates the existing InterpolationSyntax instead of creating a new one: the expression is replaced by nameof(...) and the alignment clause is kept.
• The format clause is still removed, as before. This is safe because the analyzer only reports interpolations whose format produces the name of the value (G, g, F, f, or none).
• Added InterpolatedString_Alignment test covering positive and negative widths, with and without a format clause, plus a non-reported :D case.

Validation

• The new test fails without the fix and passes with it.
• ReplaceEnumToStringWithNameofAnalyzerTests pass on roslyn4.8, 4.14, 5.0, 5.6 and 5.9 (15/15 each).
• dotnet run --project src/DocumentationGenerator produces no changes.