feat: per-platform packageContents for multi-file sub-packages

[dsherret]

Summary

Some process plugins ship more than a standalone binary — e.g. a self-contained .NET app where the executable references DLLs and a native runtime in the same directory (dprint-plugin-roslyn extracts to ~205 files). The new optional packageContents per-platform field lets the helper ship the whole directory instead of just one file.

```ts
platforms: [{
platform: "linux-x86_64",
binaryPath: "./out/linux-x64/dprint-plugin-roslyn",
packageContents: "./out/linux-x64",
}]
```

When set:

• The entire directory is copied recursively into the sub-package, preserving nested layout.
• `binaryPath` must be a file inside `packageContents` (validated). Its basename still names the executable in the npm reference URL, and it still gets `chmod 0o755` on Unix.
• The per-platform checksum in `plugin.json` is the sha256 of the resulting tarball, so it covers every shipped file — not just the executable.

When `packageContents` is unset: behavior is unchanged (single-file copy).

Test plan

• `deno test -P` — 9/9 pass, including:
  • new "packageContents ships the whole dir" test that asserts executable + multiple DLLs + a nested sub-directory file all land in the sub-package, and that plugin.json's checksum matches sha256 of the resulting tarball
  • new validation test that rejects a `binaryPath` outside the supplied `packageContents` directory
• `deno check` / `dprint check` clean.
• After release, will be used by `dprint-plugin-roslyn`'s upcoming `create_npm_packages.ts`.