Support projects without a 'package.json'

[bryant-finney]

Problem Statement

prettier supports many languages other than JavaScript (YAML, Markdown, JSON, HTML, etc.). Integrations with tools such as pre-commit or vscode make it convenient to use prettier in repositories that don't depend on node.js or define a package.json.

However, this action fails if package.json does not exist in the repository.

Example

See https://github.com/bryant-finney/demo-reviewdog-action-prettier/pull/1:

• the first job in its 🌈 Prettier workflow fails to add comments to the PR:

  npm ERR! enoent ENOENT: no such file or directory, open '/home/runner/work/demo-reviewdog-action-prettier/demo-reviewdog-action-prettier/package.json'

• the second job validates a fix: https://github.com/bryant-finney/demo-reviewdog-action-prettier/pull/1#discussion_r1428876483