RFC: update slugify for gatsby-plugin-page-creator

[cpboyd]

With the release of ESM support, I updated all of my libraries including slugify.

When transitioning from my own slug usage to File System Route API, I noticed a difference in handling that was fixed by sindresorhus/slugify#71

I'm not sure if there's a specific reason to avoid updating plugin dependencies to ESM versions, especially given that @sindresorhus/slugify v1 hasn't seen any updates in almost 3 years: https://www.npmjs.com/package/@sindresorhus/slugify?activeTab=versions