Add engines field for ds

[1stvamp]

Change Type

• Feature
• Chore
• Bug Fix

Change Level

• major
• minor
• patch

Further Information (screenshots, bug report links, etc.)

As per the readme, the ds CLI only runs in NodeJS v12, so this change adds a engines.node to ds's package.json restricting it to >=12, and uses the NPM_CONFIG_ENGINE_STRICT environment variable in misc/install.sh to temporarily enforce strict engine checking in npm when installing the client.

My reasoning here is that when pulling down the client users may be installing onto older linuxes (such as Ubuntu 16.04) with ancient node versions that need PPAs or other sources to get v12 installed, and that by allowing the install to progress without any warnings could just result in lots more erroneous bugs and general confusion.

Checklist

• Added tests / did not decrease code coverage
• Tested in supported environments (common browsers or current Node)

[ceejbot]

Eeeexcellent. Addresses a footgun.