Add support for package.json5 format

[kokoichi206]

Summary

This PR adds support for package.json5 format when reading the packageManager field, making this action consistent with pnpm's own support for JSON5.

Note:
I'm submitting this PR directly with implementation instead of opening an issue first, as the change is small and straightforward. I hope this approach is acceptable
- please let me know if you'd prefer to discuss this in an issue first!

Motivation

According to pnpm's documentation:

In addition to the traditional package.json format, pnpm also supports package.json5 (via json5) and package.yaml (via js-yaml).

This action already supports package.json and package.yaml, but not package.json5. This PR adds the missing support to be consistent with pnpm itself.

Changes

1. Added json5 package as a dependency
2. Updated parser logic to use JSON5.parse() instead of JSON.parse() for non-YAML files
3. Updated README to document package.json5 support

Note:
Since JSON5 is a superset of JSON, the JSON5.parse() can parse both standard JSON and JSON5 formats.
This means existing package.json files continue to work without any changes
- this is a fully backward-compatible enhancement.