feat: Add support for Stack Snippets

[threefjefff]

Add Stacks Snippet support to the editor as an external plugin.

• Button to open an external modal, passing existing Stack Snippet if selected
• Added to Prosemirror and Markdown IT schemas, so that snippets are correctly parsed and serialized between the two types of Node/Token streams.
• Added a paste handler to convert pasted snippet code directly into the rich text view
• Added the ability to append new content (expect to be used by the external modal)
• Added a snippet view, which is used to render the blocks of snippet content, and a "Run Code" button to render the content to IFrame
• DEVX: added a route for snippet contents such that the render button uses the webpack proxy to POST to the Stacks Snippets service when using the test harness.

PR Checklist

• All new/changed functionality includes unit and (optionally) e2e tests as appropriate
• All new/changed functions have /** ... */ docs
• I've added the bug/enhancement and other labels as appropriate

Environment(s) tested

• Device: desktop
• OS: Windows
• Browser Chrome
• Version 134.0.6998.37

Additional context

This is the baseline for Stack Snippets:

• The buttons will not appear unless a renderer option is supplied (which it is not, currently)
• The button use a placeholder icon, this is going through internal design at the moment
• Continuing to do internal UX review and squashing any bugs found. Intention is to triage and roll forwards with bugs that aren't showstoppers.

[threefjefff]

After discussion, we're going to amend the external plugin architecture to make this work as an external plugin. This also means moving the /plugins folder to the top level, and we can think about distrubtion strategies at a later date.

[giamir]

Thanks @threefjefff for implementing this integration. I think we are very close to be able to merge. The main ask I have is to make explicit the parts of the "core" editor which we rely on in the plugin. This should help us greatly if we will eventually extract the plugin and in general with evolving the core parts with confidence. Being able to see at a glance what is exposed. The comments scattered through the PR are all related to this point.

Apart from that I also have a couple of suggestions regarding the demo site:

• I would recommend to create a brand new page dedicated to showcasing the stacks snippets plugin. It is big enough that deserves it. It will also make it easier to test/try by having some prebaked examples.
• Given we will have a new page for the snippets plugin I would recommend to turn off the plugin from any other page. This emphasize that snippets are not part of the "core" editor and need to be injected as a plugin
• Cosmetic: I would suggest to change the max width of the main element from wmx6 to wmx7 (see layout.html). This is because we have several menu items now and the horizontal scollbar creeps in, in some pages.

Let me know if my suggestions make sense. Happy to sync if necessary to discuss this further. Thanks for bringing snippets to the finish line. It is a big feat. ❤️

[giamir]

Thanks @threefjefff for addressing my comments. Let's merge it! 🙂