fix: show a migration error when using the default export

[stipsan]

This improves the migration experience, instead of a generic error if attempting to use the default export it'll show a specific error:

import sanityClient from '@sanity/client'

const client = sanityClient()
// throws: TypeError: The default export of @sanity/client has been deprecated. Use the named export `createClient` instead

import SanityClient from '@sanity/client'

const client = new SanityClient()
// throws: TypeError: The default export of @sanity/client has been deprecated. Use the named export `createClient` instead

const {default: sanityClient} = require('@sanity/client')
const {default: SanityClient} = require('@sanity/client')

const client = sanityClient()
const client = new SanityClient()
// throws: TypeError: The default export of @sanity/client has been deprecated. Use the named export `createClient` instead

Unable to make this work without risking weird side-effects:

const sanityClient = require('@sanity/client')

const client = sanityClient()
// throws: Uncaught TypeError: sanityClient is not a function

And in TypeScript instead of a @sanity/client doesn't have a default export you'll see:

[github-actions]

🎉 This PR is included in version 5.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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