Add a UI that lets you check multiple features of your choice at once

[captainbrosset]

See this thread for more context: https://bsky.app/profile/sturobson.com/post/3lbhcqof2xs2v

Stuart wanted to check multiple CSS features at once on caniuse, and understand the level of support for these combined features.
This is similar to discussions we had on the web-features repository about computing statuses for groups, although here it would be user-chosen groups. The user would choose 2 or 3 features they care about, say nesting, starting-style, and something else, and it would then output the combined status.