Prepare v3.0.0

[ddbeck]

This PR merges the v3.0 branch into main.

It consists of:

• Demonstrate JSON Schema as source of truth for web-features data #2990
• Add move and split redirects to the schema #3000
• Add guidelines for moving and splitting features #3180
• Add consumer docs for moved and split features #3181
• Change schema string | string[] properties to string[] #3184

Fixes:

• Fixes Fix typo in file name #2484
• Fixes Spec is sometimes string, sometimes array #1981

How to try this out

If you want to try the npm package before the release actually happens, follow these steps:

1. Check out this branch.
2. Run npm install and npm run build.
3. Run cd packages/web-features.
4. Run npm pack. This generates a .tgz file in the packages/web-features directory. The archive's version number will match the latest npm release, but this does not matter for testing.
5. In your project, run npm install $archive_path, where $archive_path is the path to the .tgz file.

If you need the bare extended JSON for testing, follow these steps:

1. Check out this branch.
2. Run npm install and npm run build:extended. This generates data.extended.json in the root of the repository.

[szepeviktor]

Please remember #2484

[ddbeck]

Definitely, @szepeviktor! Fixing that is part of the milestone. Thank you!

[captainbrosset]

I'm testing the web-features-explorer project with this branch in web-platform-dx/web-features-explorer#65.

I decided to go simple and avoid touching too much code.
To do this, I'm splitting the ordinary and unordinary features into two separate data structures.

The existing code can therefore continue to work as it did before, and just handle the ordinary features.

For unordinary features, I've added a new 11ty template which only handles the moved and split features.

This template generates pages at the same URLs as before, so that no links are broken (e.g. /features/numeric-seperators will continue to exist).

The template either says "this feature has moved, see X instead", or "this feature has been split, see X, Y, and Z instead".

[LeoMcA]

Nice stuff: schema changes are compatible with our current implementation on MDN, but also gives us the opportunity to simplify things a little bit: I've opened a draft PR on our end to do that after v3 is released.