feat: support ESLint flat config

[Belco90]

Checks

• I have read the contributing guidelines.

Changes

• Following the official ESLint guide for making the plugin compatible with flat config

Context

Closes #853

[MichaelDeBoey]

I've seen multiple ESLint plugins that support both flat config & .eslintrc files, which could be a great upgrade path for people who can't use ESLint v9 (yet)

[Belco90]

@MichaelDeBoey Yes, planning to add compatibility with RC config finally, but will be a breaking change anyway (the configs will have a -legacy suffix as suggested in ESLint flat config migration guide).