feat: add module for package.json

[tqma113]

[Lucifier129]

farrow-schema has its subpath

• farrow-schema/validator
• farrow-schema/formatter

So exports in package.json will be needed.

If we need a real esm module in node.js, I prefer .mjs extension like what remesh do (build.esm.js).

Note: Make sure it can run esm module in node.js natively.