deps: update npm to 6.12.0

Update npm to 6.12.0

Now `npm ci` runs prepare scripts for git dependencies, and respects the
`--no-optional` argument.  Warnings for `engine` mismatches are printed
again.  Various other fixes and cleanups.

PR-URL: https://github.com/nodejs/node/pull/29885


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Christian Clauss <cclauss@me.com>