[nextjs] Update the withPigment API to accept config

[brijeshb42]

through pigment key from the nextConfig object itself instead of as a 2nd argument to the call. This is a standard approach followed by other nextjs plugins

Fixes: #83
Fixes: #88

• I have followed (at least) the PR section of the contributing guide.

[mnajdova]

I am 👌 with this change, we just have to make sure we document it as a breaking change in the changelog

[brijeshb42]

I'd say it deprecates the current API as it can still take the input as second argument. But I'll update the docs to note this.

[siriwatknp]

I am dropping my thought here based on my user expectation and writing docs.

I would like all bundler plugin to use the same name, e.g. createPigmentPlugin:

Next.js

import { createPigmentPlugin } from '@pigment-css/nextjs-plugin';

const pigmentPlugin = createPigmentPlugin(…pigment config);

export default pigmentPlugin(…next config)

Vite:

import { createPigmentPlugin } from '@pigment-css/vite-plugin';

const pigmentPlugin = createPigmentPlugin(…pigment config);

export default defineConfig({
  plugins: [pigmentPlugin()]
})

With the above signature, writing docs will be a lot easier because we can refer to the setup once and later refer to only "pigment config" for other feature.

[brijeshb42]

Seems like in latest versions of Next.js, it warns users when it finds an unknown key in the config

 ⚠ Invalid next.config.mjs options detected: 
 ⚠     Unrecognized key(s) in object: 'pigment'
 ⚠ See more info here: https://nextjs.org/docs/messages/invalid-next-config

Now, I am not so sure that we should do this.