fix: configs in plugin declaration file

[henrist]

Fixes #427

This also fixes the globals values of env config. It was previously writeable but the correct is writable (all though I don't understand why it's specified as writable in the first place). This was not type checked previously since there are no type constraints with Object.assign.

Side note: I'm noticing that the CI pipeline don't typecheck the code. pnpm tsc fails on main for me.

[Sparticuz]

If I'm not mistaken, this means we can now do this?

  {
    extends: [vitest.configs.recommended],
    files: ["tests/**", "test/**"],
    languageOptions: {
      globals: {
        ...vitest.environments.env.globals,
      },
    },
  },

instead of

  {
    files: ["tests/**", "test/**"],
    languageOptions: {
      globals: {
        ...vitest.environments.env.globals,
      },
    },
    plugins: {
      vitest,
    },
    rules: {
      ...vitest.configs.recommended.rules,
    },
  },

[henrist]

@Sparticuz I think I would have done this for something like that:

export default tseslint.config(
  // ...
  {
    ...vitest.configs.recommended,
    ...vitest.configs.env,
    files: ["tests/**", "test/**"],
  },
)

The extends property doesn't behave the way you think (I believe), and it's actually no longer part of core ESLint. See the comment here: https://github.com/typescript-eslint/typescript-eslint/blob/e44a1a280f08f9fd0d29f74e5c3e73b7b64a9606/packages/typescript-eslint/src/config-helper.ts#L22-L62

If you don't depend on the files filter it can even be top level.

export default tseslint.config(
  // ...
  vitest.configs.recommended,
  vitest.configs.env,
)

I'm new to the flat config myself though.