Sketch 50 Beta TextStyles remove .addSharedStyleWithName_firstInstance
Sketch 50 Beta TextStyles remove .addSharedStyleWithName_firstInstance

How to reproduce:
```
TextStyles.create({context: context, clearExistingStyles: true}, styles);
```

Sketch version: 50

Log:

TypeError: context.document.documentData().layerTextStyles().addSharedStyleWithName_firstInstance is not a function. (In 'context.document.documentData().layerTextStyles().addSharedStyleWithName_firstInstance(name, textStyle)', 'context.document.documentData().layerTextStyles().addSharedStyleWithName_firstInstance' is undefined)
