Add custom snippet path support

[coderberry]

Description

I would like to be able to have Marimo include a custom path to snippets that will load within the snippets list.

Suggested solution

I imagine this would be provided in two ways:

1. Via the pyproject.toml file:

[project]
name = "marimo-example"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.11"
dependencies = []
custom_snippet_path = "./_snippets"

2. Via an ENV variable:

CUSTOM_SNIPPET_PATH=./_snippets marimo edit

This path would then be added to the snippet_files method in marimo/_snippets/snippets.py file:

def snippet_files() -> Generator[str, Any, None]:
    root = os.path.realpath(
        str(import_files("marimo").joinpath("_snippets").joinpath("data"))
    )
    for _root, _dirs, files in os.walk(root):
        for file in files:
            if file.endswith(".py"):
                yield os.path.join(root, file)

Alternative

I have considered using metaprogramming to override this method, but a core solution would be much better.

Additional context

This will allow for massive adoption within my dev team and would allow us to share knowledge and snippets across different notebooks that are all within the same marimo edit server

[mscolnick]

Looks great! we can definitely start with option 1) as the config. and we can put it under [tool.marimo] like we do for our other configuration.