Add Handler Skill

Create a new handler in src/birch/handler/ following the project pattern.

Steps

1. Create the handler file: src/birch/handler/{argument}.gleam

   • Use console.gleam or json.gleam as a reference
   • Import required modules: birch/handler, birch/record, birch/formatter

2. Implement the handler:

   • Create a public handler() function that returns handler.Handler
   • Use handler.new() with:
     • name: A descriptive name (e.g., "{argument}")
     • write: Output function (fn(String) -> Nil)
     • format: Formatting function using formatter.Formatter type