feat: support a .pkgrc configuration file

[robertsLando]

Currently, pkg reads configuration from the "pkg" field in package.json or a custom JSON file passed via --config. This requires either polluting package.json with pkg-specific fields or maintaining a separate .config.json file that must be explicitly referenced on every invocation.

Proposal

Support a .pkgrc (or .pkgrc.json / pkg.config.js) file that is auto-discovered in the project root, similar to how tools like ESLint, Prettier, and Babel handle configuration. This would:

• Reduce CLI verbosity — no need to pass --config every time
• Keep package.json clean
• Allow comments and richer config formats (JS/YAML) if desired

Scope

• Define the lookup order (e.g. .pkgrc → .pkgrc.json → pkg.config.js → package.json#pkg)
• Update lib/index.ts config resolution logic
• Document the new config file format

Part of #235