Build dual ESM/CJS output with two tsc runs

[matejos]

Closes #243.

The package was CommonJS only. It now ships both, selected through the exports map. The public API does not change.

pnpm run build runs tsc twice, into dist/cjs and dist/esm. Each output directory gets a small package.json that declares its module type. That is what lets one set of .d.ts files serve both, so the repo root stays CommonJS.

Every relative import in src now carries a .js extension, and so do the lodash deep imports. ES modules do not resolve an extensionless path, and tsc emits the specifier as written. Jest maps the extension back through a new moduleNameMapper. scripts/verify-dist.ts guards it in CI by loading both builds in a real Node.js process.

The built files moved, so main is now dist/cjs/node-index.js. Any consumer that deep imports @api3/commons/dist/..., or that uses the paths override from the README, needs a path update. Named imports from the package root are unaffected.

Tested against oev-searcher, purinta-lending and airnodehub by packing the build into each: all typecheck, build and test clean, with no source changes needed. publint and @arethetypeswrong/cli are both green.