Add option to display rulers in notebook cells

[bluginbuhl]

Description

As a user, I'd like to be able to display vertical rulers in my notebook code and markdown cells so that I can easily conform to line-length requirements for linters.

The rulers should be customizable (color, position).

This feature exists in Jupyter Lab already.

Suggested solution

I was able to achieve this with a custom CSS file:

/* Vertical ruler for Marimo cells */

:root {
    --ruler-position: 88ch;
}

.cm-editor {
    position: relative;
}

.cm-editor::before {
    content: "";
    position: absolute;
    top: 0;
    bottom: 0;
    left: var(--ruler-position);
    width: 1px;
    background-color: rgba(22, 107, 86, 0.3);
    pointer-events: none;
    z-index: 10;
}

Are you willing to submit a PR?

• [ ] Yes

Alternatives

No response

Additional context

No response