Add configFileMode option

[ncallaway]

This is a proposed implementation for #157. This adds a mode option, which is passed through to the underlying call to fs.writeFileSync or atomically.writeFileSync.

The default mode here (0o666) is the default mode for both fs.writeFileSync and atomically.writeFileSync so this should be a backwards-compatible change.

[sindresorhus]

Suggested change

	mode: number | string;
	configFileMode: number;

I'm aware the underlying API supports a string, but that's only for legacy reasons. I prefer to keep it simple.

[sindresorhus]

Linkify mode to a website that explains what mode is. Not every user is familiar with the concept.

[sindresorhus]

The docs should explain when it would be useful. Most devs would not need this, and from experience, setting mode can cause a lot of problems. For example, user run a tool with sudo and then later not with sudo and then the config file is not readable, etc. I did the mode thing in a different config library, and it caused endless of support requests.

For example, on macOS, conf by default, stores the config in ~/Library/Preferences, which is not world writable. So this would not be needed if you target only macOS.

https://github.com/sindresorhus/env-paths#pathsconfig

[sindresorhus]

You need to document it in the readme too.

[ncallaway]

I'll make the changes requested, and the CI formatting fixes shortly. The only question I have is the proper destination for the mode link.

The node documentation for fs.open() (where they describe mode in the most detail) links to https://man7.org/linux/man-pages/man2/open.2.html . Another option would be the wikipedia page for File system permissions numeric notation section (https://en.wikipedia.org/wiki/File-system_permissions#Numeric_notation), or other more casual explainer: https://docs.nersc.gov/filesystems/unix-file-permissions/.

Do you have any preferences for the level of detail in the explanation link for the mode options?

[ncallaway]

I've pushed a change as a separate commit that does the following:

• Renames the parameter from mode to configFileMode
• updates the type definition to only allow number for configFileMode
• updates the documentation of mode with more details (including a link to the wikipedia link above https://en.wikipedia.org/wiki/File-system_permissions#Numeric_notation)
• adds the mode documentation to readme.md
• updates the formatting to pass npm run test

This is a separate commit to make it easy to see the changes from before, but I'm also happy to squash into a single commit before this is merged.