Specify node platform in esbuild options

[ymichael]

Currently, this defaults to "browser" (see: https://esbuild.github.io/api/#platform)

• Which then defines process.env.NODE_ENV to development OR production based on the minification option.
• Specifying platform=node leaves this alone so you can do NODE_ENV=XYZ ts-eager foo.ts

Test plan:

• Added a unit test to make sure that process.env.NODE_ENV is left a lone
• test fails without the fix in register.js
• npm run test, npm run lint, npx tsc, npx prettier --check .