Chore: Move to eslint flat config

[nickytonline]

Upgrade to ESLint 9.x.

Things to be done:

• https://eslint.org/docs/latest/use/configure/migration-guide#using-eslintrc-configs-in-flat-config
• .eslintignore is no longer supported. Need to include it in the configuration now.

See https://github.com/nickytonline/collabiano/issues/142 for more context. Collabiano was created from this template projec.t

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[heysolomon]

.take