Create sample backends for calculations and make separate backend/frontend structure in the repo.

- Currently all the calculations are done on the client side in ts/react, at some point I'd to move these to a proper backend language.
- I'm not sure what I'd like to choose to build on, but Id like to create a simple POC for at least .NET, Go, Rust, or even Next tRPC.
- Not sure what I will end up using, but would like to leave sample examples in the repo for anyone who happens to be trying to learn and comes across this repo in the future.
- Could be REST or RPC
- Leave the current CompoundInterestCalculator.tsx and create one to pull data from the backend API.
- Don't change App.tsx to use these new backend and Components, I'll leave that implementation for when the infrastructure for it is in place, since I'm currently hosting on Cloudflare pages free ties for calc.trahan.dev.