Refactoring main export from noir_js to aggregate different Noir steps

[signorecello]

Part of the first noir_js review:

I need to import Backend but Backend is literally a copy of noir-starter noir.ts file, plus it’s nested within a test folder.

import { Backend } from '@noir-lang/noir_js/test/backend/barretenberg'

I'm suggesting that we:

• Should put Backend in the src folder
• Should export Backend in the package.json file
• The Backend file is what Noir_js should be, aggregating circuit compilation, witness generation, and the backend. Currently it is literally just a wrapper on barretenberg. At least witness generation would be necessary to include.

[kevaundray]

Closing this as I believe its completed -- if this is incorrect, feel free to reopen