feat: explicitly include type

[benmccann]

I've read through several issues in both npm/cli and npm/rfcs and there seems to be universal agreement that adding "type": "commonjs" would be an improvement as it's better to explicit than implicit both for understandability and to improve performance by eliminating the need to go down detection code paths.

Several people have also suggested prompting for the type field and that is much more controversial and has been decided against, so I have not done that.

References

npm/rfcs#75 (comment)

From https://nodejs.org/en/blog/release/v20.10.0:

Detection increases startup time, so we encourage everyone—especially package authors—to add a type field to package.json, even for the default "type": "commonjs".

https://publint.dev/ and the publint package also validate that the type field is present and warn package authors if it is not. It would be best to create a package.json without warnings out of the box: https://github.com/bluwy/publint/blob/master/site/rules.md#use_type