The special names op_Exponent and op_Exponentiation are not reserved for interop.

**Describe the bug**
The special name `op_Exponent` is not reserved for interop, although Roslyn reserves and uses it.
https://github.com/dotnet/roslyn/blob/main/src/Compilers/Core/Portable/Symbols/WellKnownMemberNames.cs

The name is also recognized by System.Linq.Expressions along with `op_Exponentiation`.

It is missing from section §15.3.10.6 "Method names reserved for operators".
- https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/classes.md#153106-method-names-reserved-for-operators
- https://github.com/dotnet/csharpstandard/blob/draft-v9/standard/classes.md#153106-method-names-reserved-for-operators

**Example**
Provide an example that shows the problem in the standard. A small code snippet that should be allowed or disallowed, or code that behaves differently than the standard.

```csharp
void Main()
{
var x = new HasExponent();
var constant = Expression.Constant(x);
var exponent = Expression.Power(constant, constant);
Console.WriteLine(exponent.Method.Name);
}

class HasExponent
{
[SpecialName]
public static HasExponent op_Exponent(HasExponent left, HasExponent right) => new HasExponent();
}
```

**Expected behavior**
What should the standard declare differently?
Include `op_Exponent` as reserved.

**Additional context**
System.Linq.Expressions: https://github.com/dotnet/runtime/blob/main/src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/BinaryExpression.cs#L2756