add mobx-state-tree

[HawkinsThomas]

• Add mobx-state-tree with a simple example of managing the gamePosition in the rootStore
• Add useMst hook for quickly and conveniently accessing the root store from React components
• Add root level provider so that the whole react component tree has access to the mst store
• Refactor some functions out of the App component and into a utils directory
• Add jsconfig so that we can use absolute imports starting from the src directory
• Add .prettierrc to manage code formatting

I suggest adding the prettier extension to your VScode and making it the default formatter on-save!