Update to eslint v9 with flat config

[bmish]

This is a big change because:

• Switches to eslint flat config with eslint.config.js
• Updates all linting packages
• Updates prettier which adds some more commas
• Replaces eslint-plugin-square with some but not all of the individual lint packages/configs it provided
• Switches Jest config to typescript extension
• 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

This is part of an effort to get all dependencies updated to enable easier development. It was recently unblocked by dropping Node 16 support.