Transpile module syntax into CJS in dev

`src/index.ts` is designed to be run as an executable, and we want to be
able to run this in tests via `ts-node`. However, `ts-node` does not run
a TypeScript file when `tsconfig.json` specifies anything other than
"CommonJS" for the `module` setting ([unless you configure `ts-node`
and/or Node specially][1]). This commit instructs TypeScript to rewrite
imports using the CommonJS syntax only in the default TypeScript config
(ES6 is still used when `yarn build` is run).

[1]: https://typestrong.org/ts-node/docs/imports/

Author: mcmire

tsconfig.build.json

@@ -3,6 +3,7 @@
   "compilerOptions": {
     "declaration": true,
     "inlineSources": true,
+    "module": "ES6",
     "noEmit": false,
     "outDir": "lib",
     "rootDir": "src",

tsconfig.json

@@ -2,7 +2,7 @@
   "compilerOptions": {
     "esModuleInterop": true,
     "lib": ["ES2020"],
-    "module": "ES6",
+    "module": "CommonJS",
     "moduleResolution": "node",
     "noEmit": true,
     "strict": true,