feat: VScode/devcontainers/docs

[Rojikku]

Devcontainers allow you to spin up a docker image defined by the developers, with preset instructions, and use it as a dev environment. It's also what gets used by github codespaces. Defining one theoretically lowers the bar to entry for would-be contributors, and can allow you to write code from anywhere.
You can also finish your work on a local VScode instance, where you could do the relevant testing.

I was able to get past CORS, but I couldn't find an example of a plugin that worked even then. Mostly cloudflare errors. I assume the IP ranges are blocked by webservers.

The local dev container I have tested and works fine, but did need to add host: true because otherwise it listened on ipv6, not ipv4, which is not productive.

Notable changes:

• VScode build and debug should work properly by default
• multisrc:clean is now platform agnostic. It uses the .gitignore as a filter, and only removes ignored files under /plugins. To be extra safe, I added the -i flag, which makes it interactive, telling you what it will delete first.
• Added docs for changes and a few things while I was at it
• Decontainer config
• host: true in vite config