Add source generator

This could be improved further with source generators:

Allowing you to just decorate a partial class like so:

```c#
using Ardalis.SmartEnum;

[SmartEnum]
public sealed partial class TestEnum
{
[EnumMember]
public static readonly TestEnum One;

[EnumMember]
public static readonly TestEnum Two;

[EnumMember]
public static readonly TestEnum Three;
}
```

Generated code:

```c#
using Ardalis.SmartEnum;

sealed partial class TestEnum : SmartEnum
{
private TestEnum(string name, int value) : base(name, value)
{
One = new TestEnum(nameof(One), 1);
Two = new TestEnum(nameof(Two), 2);
Three = new TestEnum(nameof(Three), 3);
}
}
```

Start by reading the existing SmartEnum base-class implementation and the proposed partial-class usage in the issue. Define how the [SmartEnum] and [EnumMember] attributes map to generated code, then verify that the generator produces the shown constructor and members for the TestEnum example.

Written by the indexing model from the issue text.