chore(deps): Update to ESLint v9 flat config and TypeScript v5

[bmish]

This is a big change because:

• Updates all linting / typescript packages
• Switches to eslint flat config with eslint.config.js
• Replaces eslint-plugin-square with some but not all of the individual lint packages/configs it provided
• Switches Jest config to typescript extension
• Fixes new lint / typescript errors
• Note: in the future, we may be able to switch to typescript extension (eslint.config.ts): https://eslint.org/docs/latest/use/configure/configuration-files#typescript-configuration-files

Similar PR in another repo:

• Update to eslint v9 with flat config check-dependency-version-consistency#708

Follow-up to:

• chore(deps): replace eslint-plugin-square #539

Fixes #545.
Fixes #529.

[bmish]

Needed to switch to the types from json-schema, likely because of this PR affecting the typescript-eslint version:

• feat: fork json schema types for better compat with ESLint rule validation typescript-eslint/typescript-eslint#6963

[bmish]

ts-node enables us to switch to jest.config.ts.