deps: upgrade npm to 6.10.0

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Where this should land

• Node 12
• Node 10
• Node 8

Notable changes

• Faster npm install (fewer lstat calls)
• No longer possible to publish a .git folder in the root of a module
• Respect git's tagsign config on npm version-created tags if set
• Fully address (fstream security advisory](https://www.npmjs.com/advisories/886) everywhere

Changelogs

• 6.10.0