Registers a slash command /evaljs that executes JavaScript code passed through a closure or string using eval().
Examples:
/evaljs "alert('Hello, World!');"
creates an alert box.
/evaljs {: let text = "Hello, World!"; text.replace(/World/, "Universe"); :} | /echo
returns Hello, Universe! and pipes it to /echo
Based on LenAnderson/SillyTavern-EvilSlashCommand, adding closure support so you don't have to escape every single quote mark.