Pre-release: Eleventy v4.0.0-alpha.1

⛔️ This version had a publishing issue — please use v4.0.0-alpha.2 ⛔️

• Breaking: bumps Node minimum to v20 (from 18) #3886
• Breaking: removes slug filter (deprecated in v1, recommended to use slugify instead) #3893 #3858 (previous docs: https://www.11ty.dev/docs/filters/slug/)
• Breaking: removes --to=ndjson CLI feature #3382 #3858
• Breaking: Nunjucks shortcode default argument value is now undefined (previously: an empty string) in #3788 #3822
• Fork gray-matter dependency #3892 #3858
• Adds @11ty/client package #3858 #3836
• Upgrade chokidar from v3 to v4 #3442
• Swap luxon for @11ty/parse-date-strings in #3587 #3901
• Throw error when virtual template format is invalid by @dave-kennedy in #3509
• Add support for "*" selector to get all data in eleventyConfig.dataFilterSelectors by @KonnorRogers in #3904 #3903
• Migrate to flat ESLint flat config and remove cross-env by @outslept in #3758
• Add indicator for configuration-less runs by @vrugtehagel in #3799 #3654
• Fix #2798: deleting a file trigger a build in watch mode. by @aurmartin in #3482 #2798
• Update README.md with new Mastodon profile by @mijndert in #3907
• Cleaner programmatic API constructor arguments #3880
• Use error.cause for cleaner error messaging #3582

Thank you @dave-kennedy @KonnorRogers @outslept @aurmartin @vrugtehagel and @mijndert!

Milestone: https://github.com/11ty/eleventy/milestone/46?closed=1
Full Changelog: v3.1.2...v4.0.0-alpha.1