ESM-only build with TypeScript 7, drop CommonJS - #30

Keeps the unreleased v0.4.0 work going with a build modernization (all folded into the v0.4.0 CHANGELOG section since 0.4.0 is not released yet):

• Updates TypeScript 5.0.4 to 7.0.2 (latest; requires Node >=16.20)
• Drops CommonJS: ESM-only package (\import\ only). Removes the \commonjs/\ build, the
  equire\ export condition, and the CJS
  equire()\ spec
• Consolidates to a single build: deletes \ sconfig.base.json\ / \ sconfig.commonjs.json; \ sconfig.json\ is now self-contained (target ES2022, module ESNext). TS 7 no longer accepts the old ES3 target, which forced the baseline move
• Minimum Node.js is now >=16.20.0\ (\engines\ in package.json + lockfile)
• Docs: README Requirements section + ESM-only usage, CHANGELOG v0.4.0 Build/Breaking Changes entries
• Also includes the retroactive v0.3.2 engines correction (>=0.6.19 to >=12.20.0) with apology

Verified: \ sc -p .\ clean,
pm test\ passes (32 specs, 0 failures).

Note: breaking changes (CJS removal, Node floor bump) — fine to ship under the unreleased 0.4.0.