chore: fix failing tests

[Trott]

Two tests are written that require ESLint@7, but the project specifies
ESLint@6. Move to ESLint@7.

BREAKING CHANGE: ESLint@7 drops support for Node.js 8.x.

[eslint-deprecated]

Hi @Trott!, thanks for the Pull Request

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[Trott]

(Fixed the issues flagged by the bot and force pushed. This is ready to be reviewed.)

[btmills]

Hi @Trott, thanks for pointing this out! This plugin is compatible with the ESLint v6 processor API, and most of the unit tests use that. The examples use the friendlier ESLint v7 processor API. As a way to verify both work, we run the examples as an additional test step. CI runs the install-examples npm script before npm test so that the examples have ESLint v7 installed. However, I see npm run install-examples is missing from the contributing docs, so git clone ...; cd ...; npm install; npm test will fail. Any ideas?

Ideally there'd be a way to automatically run npm install for the examples as part of a local npm install but not when the plugin is installed as a dependency. Then the separate install-examples step wouldn't be necessary. Maybe we could abuse the prepare script for that? Worst case we can add npm run install-examples to the contribution section at the bottom of the readme, but something automatic would be much better.