Heroku buildpack for Bun.js
Largely copied from https://github.com/chibat/heroku-buildpack-deno
Pin a certain Bun version with a runtime.bun.txt or runtime.txt with e.g. v0.5.9 in it.
Be aware that Heroku doesn't use a new enough version of the Linux kernel to support io_uring, which is needed for Bun.write(). Use node:fs.writeFile() instead.