chore: set up ESLint and hook it up to lint-staged

[bhongy]

• set up ESLint to work with prettier and typescript using the project tsconfig
• enable ESLint on src and test folders for js, ts, tsx files in @formspree/core and @formspree/react
• fix lint errors
  • use explicit type import
  • prefer const over let
• suppress problematic lint errors that require larger refactoring
• leave lint warnings like @typescript-eslint/no-explicit-any, @typescript-eslint/no-unused-vars, and react-hooks/exhaustive-deps to be fixed in the future (keep this PR from being even larger)

Note: similar to prettier, since we run lint on lint-staged, we're not running it in CI (redundant).

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Latest commit: 08830814c215c82d925477566cb5596d9f2bd34d

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Name	Status	Preview	Comments	Updated (UTC)
formspree-react-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 28, 2023 8:20am

[colevscode]

Just checking: we're still targeting es5 so this shouldn't change the distribution, right?

[bhongy]

Just checking: we're still targeting es5 so this shouldn't change the distribution, right?

Correct, we don't change the compilation target in this PR.