triagebot: enable shortcut functionality

This allows PR reviewers and authors to easily use:

```
@rustbot ready   # S-{waiting-on-author,blocked} -> S-waiting-on-review
@rustbot author  # S-{waiting-on-review,blocked} -> S-waiting-on-author
@rustbot blocked # S-waiting-on-{author,review}  -> S-blocked
```

Author: jieyouxu

triagebot.toml

@@ -49,6 +49,11 @@ add_labels = ["S-waiting-on-review"]
 # Documentation at: https://forge.rust-lang.org/triagebot/review-changes-since.html
 [review-changes-since]
 
+# Allow reviewers and authors to easily flip between {`S-waiting-on-review`,
+# `S-waiting-on-author`, `S-blocked`}.
+# Documentation at: <https://forge.rust-lang.org/triagebot/shortcuts.html>
+[shortcut]
+
 # ------------------------------------------------------------------------------
 # PR assignments
 # ------------------------------------------------------------------------------