feat(arborist): add toJSON/toString methods to get shrinkwrap file contents without saving

[ljharb]

This allows me to build a tool that gets the lockfile's contents without touching the filesystem.

[isaacs]

Seems like a toJSON method should return an object, not a string, so that JSON.stringify(shrinkwrapObject) will return the JSON representation of the shrinkwrap object, rather than the JSON representation of the JSON string itself.

[ljharb]

ah, good call.

given that what i need is the formatted string, do you have a thought for a better choice of method name? I could also make toJSON return an object, and then have another method that calls that, and formats it, and then have save call that, but that seems like overkill.

[ljharb]

k, went with toString :-) updated!