Nano 12 - make the library ESM-only. Exchange all "require"s for "import"s.

[glynnbird]

Overview

Preparation work for Nano 12.

• Making the library ESM-only - remove any "require" use and replace with equivalent "import".
• version 12.
• Update README code to eliminate "require".
• Remove a bunch of legacy Nano code that dates back to the request/axios days. It's a bit simpler to read now with the "relax" method making the request and handling the response.
• The ..AsStream variants of the functions are now "async" - i.e. they return a Promise which resolves to a stream, rather than synchronously returning a dummy stream as before. This makes the code easier to maintain, but is a breaking change.
• Use rolldown to build CommonJS and ESM variants of the code. The module & main keys in package.json make it such that the package "just works" for both CJS and ESM code.
• Use an npm prepublishOnly script to ensure that the transpiled artifacts are generated during an npm publish event, but are gitignored from the repo.

Testing recommendations

All the tests have changed in the require-->import sense, but are functionally identical.

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;