Use player-set currency for configuring money goals

Currently, the in-game configuration for the script goals use pound sterling for the money-based goals (which is then converted to the player-set currency). The player-set currency should be used in the actual config. However, this may not even be possible, since it does not seem like retrieving the currency from the game options is possible in the GS API.

See [shortcomings #4 in the readme](https://github.com/Arastais/GlobalObjectiveSystemGS/blob/main/README.md#shortcomings--possible-bugs) for clarification.

Start with the README's “shortcomings & possible bugs” section, especially item #4, then inspect the OpenTTD GameScript API documentation for access to the game options currency. Done means money-goal configuration uses the player-set currency, or the limitation is documented if the API cannot provide it.

Written by the indexing model from the issue text.