Add Calva Connect Sequence

[PEZ]

When using Calva users sometimes have troubles getting the right dependencies in place. For this we usually recommend using Calva Jack-in, which starts the REPL, injecting the dependencies that Calva needs.

This PR makes starting the app and connecting Calva a short process like so:

0. Open the project in a VS Code window.
1. Issue the VS Code command: Calva: Start a Project REPL and Connect (aka Jack-in)
2. In the Project type menu, choose Electric Starter App
3. Soon the Calva Output window will print 👉 App server available at http://0.0.0.0:8080

Without this PR the process is:

0. Open the project in a VS Code window.
1. Issue the VS Code command: Calva: Start a Project REPL and Connect (aka Jack-in)
2. In the Project type menu, choose shadow-cljs
3. In the Launch build menu, choose :app
4. In the Connect build menu, choose :app
5. Open src/user.clj and load it in the REPL (ctrl+alt+c enter)
6. Evaluate the (main) form at the bottom of the file
7. The Calva Output window will print 👉 App server available at http://0.0.0.0:8080

I think the README should have instructions for starting the project in the editor. But as there was nothing about this there, it is maybe weird to only have Calva instructions? I could always add it, and other people can add sections for other editors. Let me know what you think.