feat: support multiple bin entries from package.json

[robertsLando]

When package.json declares bin as a string, pkg builds a single executable. When it's an object ({ "foo": "./foo.js", "bar": "./bar.js" }), pkg silently picks only one entry — the one matching the package name, falling back to the first key:

// lib/index.ts
if (bin[inputJsonName]) {
  bin = bin[inputJsonName];
} else {
  bin = bin[Object.keys(bin)[0]]; // TODO multiple inputs to pkg them all?
}

The TODO has been in the code for years. Packages with multiple CLIs (e.g. a server + a migration tool) currently need multiple invocations with --config overrides, which defeats zero-config auto-discovery (#238).

Proposal

When bin is an object and the user hasn't narrowed it via CLI, build one executable per entry. The binary name defaults to the object key; the output path defaults to outputPath/<key> (or the current --output template, with {name} substituted per entry).

{
  "name": "my-tool",
  "bin": {
    "my-tool":       "./cli.js",
    "my-tool-admin": "./admin.js"
  },
  "pkg": { "targets": ["node22-linux-x64"], "outputPath": "dist" }
}

→ dist/my-tool-linux, dist/my-tool-admin-linux.

Selecting a subset

Keep the single-binary path working for users who want one entry:

• pkg . with object-form bin → all entries (new default)
• pkg . --bin my-tool-admin → only that one
• A top-level bin string on the CLI still refers to one entry

Legacy behavior (picking the first/name-matching entry) becomes opt-in via explicit --bin <key>.

Scope

• Extend config resolution in lib/index.ts to accept an array of (name, entryPath) pairs instead of a single inputBin
• Loop the existing single-binary build pipeline per entry; share the walker/bytecode cache across entries where paths overlap to avoid re-walking node_modules N times
• Clarify output naming: {name} in --output templates should expand to the bin key, not the package name
• Interaction with .pkgrc (feat: support a .pkgrc configuration file #238): a pkgrc may override bin entirely (string, array, or object); bin still comes from package.json when the pkgrc doesn't declare one
• Document migration for users who relied on the old "first key wins" behavior

Out of scope

• Per-entry targets / outputPath — keep the shared config shape for this pass
• A new hooks surface (feat: build hooks (pre/post + per-file transform) #252) already covers per-binary postBuild

Part of #235