npm / arborist / #252

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fix: package-lock.json custom indentation

package-lock.json files should follow the same indentation used on
package.json files.

This commit fixes the issue by forwarding the proper indent selected for
package.json to Shrinkwarp.save method. Also, added tests asserting
appropriate tab-indentation for both files.

Fixes: https://github.com/npm/cli/issues/1662

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