Relates-to: #14391

npm cache dir was added in #14391, but the command does not exist from v6 (released in 2018) to v11 (latest).
https://docs.npmjs.com/cli/v11/commands/npm-cache

This PR changes it to npm config get cache.

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known): v10/v11.