feat: add Type Modifiers > Prickly Predicates appetizer

[JoshuaKGoldberg]

PR Checklist

• [x] Addresses an existing open issue: fixes #77
• [x] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

Adds a second appetizer to Type Modifiers with a cowboy/southwest/western theme. 🤠

I'd originally meant for this to be a third step in the existing Modifiers of the Type appetizer, but found that the content worked better on its own. This way we can explore a few more pieces of type predicates: a keyof typeof and an array filter.

💖

[JoshuaKGoldberg]

Compile failures are unrelated.