fix: runnable via npx

In the switch to esbuild I broke the ability to run via npx. esbuild was
producing files in the esm format, but node wasn't happy with that so
I've switched it to commonjs. Also the keys weren't being bundled
anymore either.

Fixes jagregory#34

package.json

@@ -9,7 +9,7 @@
     "lib"
   ],
   "scripts": {
-    "build": "rm -rf lib && tsc --emitDeclarationOnly --project tsconfig.build.json && esbuild $(find src -name '*.ts') --outdir=lib --platform=node --target=node14",
+    "build": "rm -rf lib && tsc --emitDeclarationOnly --project tsconfig.build.json && esbuild $(find src -name '*.ts') --outdir=lib --platform=node --target=node14 --format=cjs && cp -r src/keys lib/keys",
     "integration-test": "jest --config integration-tests/jest.config.js",
     "integration-test:watch": "jest --config integration-tests/jest.config.js --watch",
     "lint": "eslint src/**/*.ts && tsc --noEmit",