feat: add Yarn Berry (v2+) support with automatic version switching

[ddusht]

Summary

Adds support for Yarn Berry (v2+), removing the previous limitation of Classic-only installs. The install process is now selected automatically at runtime based on packageManager in package.json.

Changes

• switching_install_process.go — New SwitchingInstallProcess that reads packageManager in package.json and delegates to either the Classic or Berry process. No changes to existing YarnInstallProcess logic.
• berry_install_process.go — New BerryInstallProcess implementing the InstallProcess interface:
  • Runs yarn install --immutable (Berry's equivalent of --frozen-lockfile)
  • Sets YARN_NODE_LINKER=node-modules for traditional node_modules layout
  • Redirects install-state.gz into the layer for caching across builds
  • Supports app-provided Berry binaries via yarnPath in .yarnrc.yml (invoked as node <yarnPath> install --immutable); falls back to buildpack-provided yarn with YARN_IGNORE_PATH=1
• run/main.go — Wires up both processes through SwitchingInstallProcess.
• README.md — Documents Classic vs Berry selection logic and yarnPath behaviour.

Behaviour

packageManager in package.json	Install command
yarn@1.x.x or absent	yarn install --frozen-lockfile (Classic)
yarn@2.x.x or higher	yarn install --immutable (Berry)

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[ddusht]

paketo-buildpacks/nodejs#627
paketo-buildpacks/yarn#1062
paketo-buildpacks/samples#1400