Add warning/callout about using Bun with Railpack

[henrikvtcodes]

I was deploying a project that uses SvelteKit & Bun to Railway. When using Railpack, it kept silently failing with no additional context; the build would work, and it'd run the start command but the app would never come up.

Through some investigation, it appeared that a Bun binary from the node_modules folder was being invoked during start, as opposed to the version of Bun installed by mise via Railpack. When invoked, this binary does nothing and exits successfully.

The cause of the issue ended up being that I had bun in my package.json dependencies, and that binary took priority over the mise-installed one that would actually work.

I think there should be a warning of some sort in the Railway docs (or Railpack docs) that addresses this, as I couldn't find any existing documentation or support threads about this.