fix: tsc-esm and moving files

[jiito]

Attempts to fixes #42

Changes

1. Adding tsc-esm to @crossmint/solana-auth-base to compile typescript to esm
2. Moving all files to a flat directory structure
   • This was done because tsc-esm doesn't seem to fix directory imports

Testing plan

1. Checkout this branch
2. Run yarn build in the root directory.
3. cd packages/examples/nextjs-starter && yarn install
4. You will need to update the utils/firebaseClient and add a web config from a Firebase project
5. Also update utils/firebase to reference the correct service account.
6. yarn dev