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.

[KSXGitHub]

You should change the logic into the following:

1. If the file name ends with .yaml, use YAML.
2. If the file name ends with .json5, use JSON5.
3. Otherwise, use JSON.

IIRC, pnpm only uses JSON5 for package.json5.

[kokoichi206]

@KSXGitHub
Thank you for the review.
Fixed in be2217e.

[Copilot]

Pull request overview

This PR adds support for reading the packageManager field from package.json5, aligning this GitHub Action’s manifest parsing behavior with pnpm’s JSON5 support.

Changes:

• Add json5 as a dependency and lock it in pnpm-lock.yaml.
• Extend manifest parsing to handle .json5 files when reading packageManager/devEngines.
• Update README to document package.json5 as a supported manifest format.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 4 comments.

File	Description
src/install-pnpm/run.ts	Adds JSON5 parsing branch when reading the workspace manifest
README.md	Documents package.json5 support for package_json_file
package.json	Adds json5 dependency
pnpm-lock.yaml	Locks json5 dependency version and integrity

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

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[Copilot]

dist/index.js is the action entrypoint (action.yml), and the repo's PR workflow rebuilds and checks that dist/index.js is committed. Since this change adds a new runtime dependency (json5) and modifies parsing logic, you need to run pnpm run build and commit the updated dist/index.js; otherwise CI will fail and the published action won't include the change.

[Copilot]

PR description says non-YAML manifests are parsed with JSON5.parse() (which would cover both .json and .json5), but the implementation only uses JSON5.parse() for .json5 and still uses JSON.parse() for .json. Either update the description or simplify the logic to use JSON5.parse() for all non-YAML files (since JSON5 is a superset of JSON).

[Copilot]

The nested ternary for selecting the manifest parser is getting hard to read/extend with multiple formats. Consider refactoring to a small helper (e.g., based on path.extname(packageJsonFile) or a switch) so future additions like .yml don’t further degrade readability.

[Copilot]

README documents package.json5 support here, but action.yml’s package_json_file input description still only mentions package.json. To keep the Marketplace/action docs consistent, please update action.yml to mention package.yaml and package.json5 as supported formats too.