Sign immutadot and immutadot-lodash

[charlyx]

Prerequisites

• [x] I have read the Contributing guidelines
• [x] I have read the Code of conduct and I agree with it

Description

Sign and verify packages using pkgsign¹.

¹ https://medium.com/redpoint/introducing-pkgsign-package-signing-and-verification-for-npm-5b833e0ec2d4