add fields resolved and integrity to deptree and depgraph

[milahu]

• Tests written and linted
• Documentation written / README.md updated https://snyk.io/docs/snyk-for-node/
• Follows CONTRIBUTING agreement
• Commit history is tidy https://git-scm.com/book/en/v2/Git-Branching-Rebasing
• Reviewed by Snyk team

What this does

add fields resolved and integrity to deptree and depgraph

continue #112 by @abdulhannanali

commit 9dd6745 is based on 60853a7

i have added the new fields as node.info.labels.resolved and node.info.labels.integrity, because @snyk/dep-graph allows to add custom string values to node.info.labels

Notes for the reviewer

probably this will not be merged, because snyk does not use resolved and integrity values of lockfiles

to me, the resolved and integrity values are useful to build a custom npm package installer. for example usage, see milahu/pnpm-install-only@4278992, where i have both a tree-walker and a graph-walker

merged in https://github.com/milahu/nodejs-lockfile-parser