danger: Update PR prefix pattern (#29432)

This PR updates the Danger PR prefix pattern to allow underscores (`_`)
and spaces (` `) in the prefix.

Release Notes:

- N/A

script/danger/dangerfile.ts

@@ -2,6 +2,7 @@ import { danger, message, warn } from "danger";
 const { prHygiene } = require("danger-plugin-pr-hygiene");
 
 prHygiene({
+  prefixPattern: /^([a-z\d\(\)_\s]+):(.*)/g,
   rules: {
     // Don't enable this rule just yet, as it can have false positives.
     useImperativeMood: "off",