feat: 🧑‍💻 add recipe add-function (#2)

## Description 

I'm not sure that this is the best way to go about it - and as we,
@lwjohnst86 , talked about we might want to make a tool for tasks like
this at some point. But maybe this is fine for now?

At least I find this helpful - instead of having to create the function
file and corresponding test file by hand.

With this, the command `just add-function sprout core hello` creates
`sprout/core/hello.py` and the corresponding test file
`tests/core/test_hello.py`

## Reviewer Focus

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Author: signekb

justfile

@@ -57,3 +57,8 @@ reset-local:
 # Build the documentation website using Quarto
 build-website:
   docker run --rm -v $(pwd):/site -w /site ghcr.io/quarto-dev/quarto:latest quarto render
+
+# Add files for a new function (function file and test file)
+add-function app part name:
+  touch ./{{app}}/{{part}}/{{name}}.py
+  touch ./tests/{{part}}/test_{{name}}.py