Can we move the implementation to ./config/validation/declare-config.ts and re-export it here?

I suggest we rename it to declareConfig to align with declarePreset.

Aside: maybe we can also add an alias of declarePlugin in helper-plugin-utils to declare, and somehow we should have declarePreset providing the same typing hints as declareConfig.

The name defineConfig is inspired by eslint. I've also noticed that similar functions to tsup and vite are called defineConfig.

Understood. However we don't have to choose the same name as other projects, as different naming could also benefit users, for example, think of a @babel/eslint-parser user using eslint.config.ts:

import { defineConfig } from "@eslint/config"
import { declareConfig } from "@babel/core"
import babelEslintParser from "@babel/eslint-parser"

export default defineConfig([{
  languageOptions: {
      parser: babelEslintParser,
      parserOptions: {
        babelOptions: declareConfig({
          plugins: [ ... ],
        }),
        requireConfigFile: false,
      },
    },
}])

A different export name reduces the possibility that people misused exports from @babel/core to eslint or vice. Since both defineConfig and declareConfig are merely typing helpers, such usage may still work if users do not run tsc on the config files.