Add script to patch project with preview tarball URLs (#91057)
### What?
Adds a new script `scripts/patch-preview-tarball.mjs` that patches a
target project's `package.json` to use Next.js preview tarballs from
`vercel-packages.vercel.app`.
### Why?
When testing unreleased Next.js changes against external projects (e.g.,
reproducing bugs, verifying fixes), there's no convenient way to point a
project at a specific Next.js commit's build artifacts. This script
automates the process of wiring up preview tarball URLs into a project's
dependency overrides.
### How?
The script:
1. **Resolves a commit SHA** via one of three methods:
- `--commit <sha>` — use an explicit commit
- `--branch <name>` — look up the branch tip via the GitHub API (`gh`)
- *(default)* — use the local `git rev-parse HEAD`
2. **Builds tarball URLs** for `next`, `@next/mdx`, `@next/env`, and
`@next/bundle-analyzer` using the
`vercel-packages.vercel.app/next/commits/<sha>/<pkg>` pattern.
3. **Verifies** that the preview tarball exists (HEAD request to the
`next` tarball URL).
4. **Patches `package.json`** in the target project (or its workspace
root) by setting both `overrides` (npm/pnpm) and `resolutions` (yarn)
for the packages.
**Usage:**
```bash
node scripts/patch-preview-tarball.mjs --project <path> [--commit <sha> | --branch <name>]
```
