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".

There appears to be a lot of change, but it's a lot of cut and paste.

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;

[glynnbird]

Note to future self: It is possible to turn the ESM into CJS with:

rolldown --input lib/nano.js --file lib/dist.js --format cjs

I think ESM is the future though