feat: add exports map, sideEffects flag, and named country exports for tree-shaking

[jsumnerp]

Problem

Importing anything from stdnum currently bundles every country's validators (~205KB minified / ~50KB gzip), because the stdnum object eagerly references all of them and the package publishes no other entry points. Consumers who need a handful of validators resort to fragile deep imports into lib/esm/... internals.

This PR finishes what the exports_NOT_YET sketch in package.json started (its trailing-slash form was deprecated and later removed by Node, so it wouldn't work as-is).

Changes

1. exports map — official subpaths for every validator and country:

   import { validate } from 'stdnum/gb/nino';   // single validator
   import * as GB from 'stdnum/gb';             // whole country

   • The node condition resolves to the CJS build for both require and import in Node. This is deliberate: lib/esm is bundler-only today (the package has no "type": "module" and the ESM output uses extensionless relative imports, which Node's ESM resolver rejects), so pointing Node at it would break. Node behaviour is exactly what main gives today.
   • Bundlers (no node condition when targeting browsers) hit import → the ESM build, same as today's module field.
   • Identity passthroughs keep all existing deep imports working: stdnum/lib/esm/*, stdnum/lib/cjs/* (with or without .js), and stdnum/src/*.
   • Top-level main/module/types fields are unchanged, so old resolvers (including TS node10) are unaffected.

2. "sideEffects": false — lets bundlers drop unused modules.

3. Named per-country exports in src/index.ts — import { GB } from 'stdnum' is now tree-shakeable. The stdnum object export is unchanged for full backward compatibility.

4. README section documenting the tree-shakeable forms.

Verification

• npm test: 1611/1611 pass (under TZ=UTC; mx/curp has a pre-existing timezone-sensitive assertion that fails on non-UTC machines on main too)
• npm run lint, npm run prepublishOnly (both tsconfigs), prettier --check: clean
• Smoke-tested against the packed tarball:
  • Node CJS: require('stdnum'), require('stdnum/gb/nino'), require('stdnum/lib/cjs/gb/nino') ± .js — all work
  • Node ESM: import { stdnum, GB } from 'stdnum' and import { validate } from 'stdnum/gb/nino' — work (resolve to CJS via the node condition)
  • TypeScript: consumer type-checks under moduleResolution: bundler and node16
  • esbuild browser bundles: barrel { stdnum } 205KB (unchanged), named { GB } 8KB, subpath stdnum/gb/nino 5KB, legacy stdnum/lib/esm/gb/nino still resolves

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