Add support for interpolated strings via InterpolatedStringHandler

...so that appending an interpolated string like this:

```csharp
sb.Append($"BLAHBLAH {integer}");
```

behind the scenes this should result in this (pseudo code)

```csharp
ZString.AppendInterpolatedStringHandler handler = new ZString.AppendInterpolatedStringHandler(9, 1, zstringBuilder);
handler.AppendLiteral("BLAHBLAH ");
handler.AppendFormatted(value);
```