Upgrade to ESLint v7

[tylerbutler]

ESLint v7 has been released: https://eslint.org/blog/2020/05/eslint-v7.0.0-released

It includes a number of nice features, including free-form comments on inline lint rule disables, no need to use the --ext flag to include ts files, and many other fixes and enhancements.

@typescript-eslint, our primary dependency, should release a v7-compatible update soon: typescript-eslint/typescript-eslint#1550

We should upgrade.

---

List of dependencies that need to support v7 before we can upgrade:

• @typescript-eslint (feat: upgrade to ESLint v7 typescript-eslint/typescript-eslint#2022)
• eslint-plugin-eslint-comments (feat: Support ESLint 7.x mysticatea/eslint-plugin-eslint-comments#39)
• eslint-plugin-import (New release supporting ESLint 7 import-js/eslint-plugin-import#1772)
• eslint-plugin-no-null
• eslint-plugin-optimize-regex (consider dropping this and using the unicorn better-regex rule instead - discussion here: Join forces with eslint-plugin-unicorn BrainMaestro/eslint-plugin-optimize-regex#59)
• eslint-plugin-prefer-arrow
• eslint-plugin-react ([New] support eslint v7 jsx-eslint/eslint-plugin-react#2635)
• eslint-plugin-unicorn (Update ESLint to v7 sindresorhus/eslint-plugin-unicorn#577)

[tylerbutler]

Addressed by the following PRs:

#3754
#3769
#3770
#3790
#3791
#3792
#3793
#3794
#3795
#3851