Support ESLint v8

[bmish]

• Peer dependency expanded to both ESLint v7 and v8
• Test under both ESLint v7 and v8
• Temporarily use a fork of the dev dependency eslint-plugin-unicorn until the official ESLint v8 support is released in that plugin (same solution as in Build: test on eslint v8 (fixes #175) eslint-community/eslint-plugin-eslint-plugin#186)

Fixes #208

[platinumazure]

Thanks @bmish for the PR.

I really don't like the idea of depending on a branch that could change at any time, but we'll keep this as an option.

Any idea why some of the CI checks are failing?

[bmish]

Assuming I can get CI working, I do think we should proceed with the fork of eslint-plugin-unicorn. Why?

• Most importantly, eslint-plugin-unicorn is an internal-only dev-dependency, and we shouldn't let a non-essential dev-dependency that has no public-facing impact to our consumers block us from releasing a version of eslint-plugin-qunit that supports ESLint v8
• The fork is owned by the maintainer of eslint-plugin-unicorn
• We can pin to a specific commit of the fork branch if desired
• Other repositories are using this fork to unblock themselves while awaiting for the official fix to be released

I'm currently unsure why CI is failing and trying to debug.

Run npm ci
npm ERR! Error while executing:
npm ERR! /usr/bin/git ls-remote -h -t ssh://git@github.com/fisker/eslint-plugin-unicorn.git
npm ERR! 
npm ERR! Warning: Permanently added the RSA host key for IP address '140.82.113.4' to the list of known hosts.
npm ERR! git@github.com: Permission denied (publickey).
npm ERR! fatal: Could not read from remote repository.
npm ERR! 
npm ERR! Please make sure you have the correct access rights
npm ERR! and the repository exists.
npm ERR! 
npm ERR! exited with error code: 128

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/runner/.npm/_logs/2021-10-10T15_45_48_045Z-debug.log

[platinumazure]

We can pin to a specific commit of the fork branch if desired

Let's do this, please. Then I'll be okay with switching over to the fork (ideally after CI passes).

[coveralls]

Coverage remained the same at 100.0% when pulling 72e3a18 on bmish:eslint-8-support into a66439e on platinumazure:master.

[bmish]

Fixed CI issue using git+https URL for fork suggested in this issue: npm/cli#2610