Minimum node version requirement: >=12.0.0

[iCrawl]

Starting with v12.0.0 the minimum node version requirement for discord.js will be >=12.0.0 to make use of features like async/await, Object.entries(), Object.values() (ES2017) etc.
This will allow us to implement features like #1376 easily and without further obstacles.

Since this will be a rather big breaking change users that want to use discord.js v12.x.x will need to update their node version to at least 12.0.0. The reason why we don't plan on supporting >=7.0.0 is that --harmony is way too unreliable and unstable in versions prior to 7.6.0 (e.g. memory leaks) .

Benefits:

• Full access to ES2017 features
• • async / await
• • Object.entries()
• • Object.values()

Drawbacks:

• Dropping node 6/7/8 support

[devsnek]

for build:
now: add v8(-nightly) (and v9(-nightly) when it releases)
after v11.1 is released: remove v6/v7 builds

[iCrawl]

This is live now, forcing node 8 will be done in around 1 week or a as soon as we start using features that are not compatible with node 6 anymore.

[iCrawl]

As of 375b89a this (forcing node 8) is now implemented in master.