Custom properties like description not allowed on ESLint flat configs

[bmish]

Context:

• Change Request: Standardize documentation properties (e.g. allow configs to export description) eslint/eslint#17842

If we can't store description / meta.description / meta.docs.description on configs / processors (along with other properties like deprecated, then we'll have to allow these to be defined in our config file.

Part of:

• Support ESLint flat config #376