chore: ESLint overhaul

[artus9033]

Summary

This PR:

• upgrades ESLint from v8 to v9
• migrates ESLint legacy config format to the new flat config style
• adds eslint-plugin-node-import to enforce node: prefixes in imports of built-in node modules
• reformats files & adapts typings to satisfy the new rules
• adds an ESLint config to lint the TesterIntegrated demo app

Test plan

CI green, yarn lint passes locally.