feature(eslint): eslint 7 upgrade

[rwaskiewicz]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Unit tests (npm test) were run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

ESLint v7.0 is not supported by this library

Relates to #32, most of this work is derived from the original work done by the author of #32. @alex-ketch did most of the work here 🎉

GitHub Issue Number: N/A

What is the new behavior?

update dependencies to support ESLint 7, and make v7 the earliest
version that is supported by this library.

update tests for rules that have fixers, but weren't being previously
evaluated. this required some test cases to be updated as well as they
were not being properly run previously.

update readme to specify cleaner install instructions for npm 7+ users

BREAKING CHANGE: ESLint version 6 and below are no longer supported

Does this introduce a breaking change?

• Yes
• No

Applications will be required to upgrade to ESLint v7.0.0. This PR does not include support for v8.0.0 of ESLint.

Testing

• Using npm 8, I spun up a new Stencil component library using npm init stencil.
• I added the following .eslintrc.json file to the project

  {
    "parserOptions": {
        "project": "./tsconfig.json"
    },
    "extends": [
        "plugin:@stencil/recommended"
    ]

}

- I added the following `lint` script to `package.json`:
`"lint": "eslint src/**/*{.ts,.tsx}"`
- Pull down this branch, `npm ci && npm run build && npm pack`
- In the sample component library, install the tarball that was just generated
- `npm run lint` to smoke test

## Other information

<!-- Any other information that is important to this PR such as screenshots of how the component looks before and after the change. -->

[rwaskiewicz]

Note to self: I need to remove or commit to this

[rwaskiewicz]

CI if failing, looks like async-methods doesn't work ATM on Windows

[rwaskiewicz]

Thanks @alex-ketch! LGTM!