Renamed module to STPluginNeon (#12)

Renamed the module `NeonPlugin` → `STPluginNeon` In order to avoid a
name collision with the main type `NeonPlugin`.
This in turn, allows for easier disambiguation for projects that also
have their own `Theme` or `Colors` type.

E.g. it is now possible to do this, even if your project has its own
`Theme` type:

```swift
import SwiftUI
import STTextViewSwiftUI
import STPluginNeon

extension STPluginNeon.Theme {
    public static let myCustomTheme = STPluginNeon.Theme(colors: [:], fonts: [:])
}
```

## Discussion
See issue #11