Add eslint-plugin-vitest

[mcous]

Overview

I've been trying out eslint-plugin-vitest in a personal project and it's been helpful. This PR adds this plugin and an opinionated configuration to the base ESLint config that takes includes the recommended config and also takes care of various bits of feedback I've given on PRs.

Adding this config to the app triggers about 70 new errors, so this will be a relatively involved upgrade, but the payoff will be more consistent, idiomatic tests.

Review requests:

Review the explicitly enabled tests for any you think are improper:

• vitest/consistent-test-filename
• vitest/consistent-test-it
• vitest/no-conditional-expect
• vitest/no-conditional-in-test
• vitest/no-conditional-tests
• vitest/no-restricted-matchers
  • Disallow toBeTruthy and toBeFalsey because they hide bugs
• vitest/no-restricted-vi-methods
  • Disallow vi.spyOn because partial mocking is harmful
• vitest/prefer-each
• vitest/require-top-level-describe
• vitest/valid-expect

[DTCurrie]

✅

[mcous]

Superseded by #35 and #36