Add Zed extension

[pierre-H]

Check https://zed.dev/docs/extensions/developing-extensions

[zdavison]

https://github.com/zdavison/zed-quint

I clauded up a port of the existing VSCode extension to Zed.

It's MIT license, so if the Quint team want to just lift it and support it officially, that's fine by me.

Here's the PR to Zed extensions repo that adds it as an officially supported extension that you can install from within Zed.