Upgrade shop beta implemented

I have uploaded an update to the project with a working prototype of the in-game upgrade store, points earned during a run can be used to purchase upgrades for subsequent runs.

The upgrade store is functional as is, but I plan to improve the visual aspect of the store by adding gfx elements such as graphics for each upgrade.

The upgrades available are those which were simplest to implement in the current game engine, I plan to add a set of consistent upgrades, such as each enemy type needing to be purchased before it appears, along with more upgrades to the player stats.

The upgrade shop also requires implementation of a persistent game state, so that the upgrades purchased do not reset each time the game is loaded, this should be fairly straightforward from what I have seen online.