Remove sbt-scalajs-bundler dependency

[rleibman]

Summary

• Removes the sbt-scalajs-bundler dependency from sbt-converter. It has no sbt 2.0/Scala 3 build and looks dormant — it was blocking a move to sbt 2.0, but sbt-converter never actually used its bundling features (JS resource hooks were already stubbed to Nil), only its npm-dependency-declaration and npm install plumbing.
• Adds a small self-contained NpmInstall task (sbt-converter/.../internal/NpmInstall.scala) that writes package.json and runs npm install/yarn install directly via os-lib, with content-hash caching to skip redundant installs.
• Renames the bundler-derived keys to new st-prefixed ones: npmDependencies → stNpmDependencies, npmDevDependencies → stNpmDevDependencies, useYarn → stUseYarn; adds stNpmResolutions and stNpmInstall.
• Updates scripted tests (sourcegen, sourcegen-mixed, japgolly) and docs to the new key names, and drops sbt-scalajs-bundler from their plugins.sbt.

This is a prerequisite refactor only — the actual sbt 2.0 cross-build (Scala 3 axis, pluginCrossBuild/crossSbtVersions, porting to sbt 2.0 APIs) is tracked separately in a follow-up PR.

Test plan

• sbt-converter/compile and whole-build Test/compile succeed under JDK 17
• sbt-converter/scripted (could not be verified in this sandbox due to a pre-existing, unrelated dependency-resolution issue reproduced identically on unmodified master; should be re-run in normal dev/CI)

https://claude.ai/code/session_01TBZdpjcwLC4C22494C7zNH

[rleibman]

Hi! @oyvindberg! I hope it's ok, I had my AI draft this PR. I'm trying to move my projects to sbt 2.0 and I don't see any work on the sbt-converter towards that, the big stumbling block is the bundler, which I don't think is maintained, so this first PR is to remove the dependency with the bundler, and having the sbt plugin reproduce the parts that it made.

[oyvindberg]

Hey, long time!

I would be ok with many solutions here, really. I don't use sbt-scalajs-bundler or even sbt myself, so most of all i would like to nuke the sbt plugin from orbit 😅

but to be less dramatic, lets do something like you propose. i guess what i want to know is if there can be a well-defined hook which users of bundlers integrated with sbt can use? one thing has the responsibility of calling npm install, then ST needs to be called, and finally the results of ST needs to be appended to the resolved deps of the build. can you have your agent look into it? maybe consider it in relation with an sbt vite plugin as well? i seem to have an impression that exists

[rleibman]

the sbt-vite plugin is also dormant, I think the recommended approach is to use vite directly, but that's for scalajs end users, if I understand correctly, what sbt-scalajs-bundler was doing here was setting up which js the converter was going to convert. So even if you recommend to end users to use vite (or sbt-esbuild, which is another option) you still need a way to figure out what to convert, and that's why replacing it with this minimal implementation seems to make sense.