Package and ESLint updates

[fsvreddit]

• Switch from lodash to lodash-es
• Update dependencies for vitest, date-fns and date-fns-tz. In particular the date-fns ones were a major version behind.
• Update ESLint config and related packages (ruleset not affected):
  • Remove legacy FlatCompat shim to a native ESLint 9 config
  • Swap out vitest plugin from the older eslint-plugin-vitest (not updated in two years) to Vitest's up to date plugin that works better with ESLint 9