chore: convert to eslint flat config internally

[bmish]

This converts to the new ESLint flat config internally (will be the default config format in ESLint v9). It does not cause any behavior change internally or externally. We still need to publicly support ESLint flat config after this.

Took some inspiration from:

• feat: support eslint.config.js eslint-community/eslint-plugin-eslint-plugin#347

Relevant info:

• https://eslint.org/blog/2023/11/whats-coming-in-eslint-9.0.0/#flat-config-now-the-default-and-has-some-changes
• https://eslint.org/docs/latest/use/configure/configuration-files-new
• https://eslint.org/docs/latest/use/configure/migration-guide

[coveralls]

coverage: 100.0%. remained the same
when pulling 9b7387d on bmish:convert-to-flat-config-1
into dac8972 on platinumazure:master.

[platinumazure]

Thanks for this, @bmish!