Add devcontainer configuration

[ran-dall]

This adds the configuration for VSCode Devcontainer and Codespaces.

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Latest commit: 5aaf4a7

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[ran-dall]

@natemoo-re Before this is ready, I think I want to get rid of nvm in favor of volta that you're already using.

[ran-dall]

@natemoo-re Restarting the work on this PR. So I replaced nvm with volta, but you still need to run volta install node && npm install --global pnpm to get everything set up. Maybe after volta-cli/volta#1273, things will get nicer, but till then, I think this is as good as it's going to get.

Otherwise, it can be merged as-is, and I'll update it later when that PR is merged.

[ran-dall]

@natemoo-re Actually, I fixed it. Now it will automatically set up node and pnpm@7.0.1 with volta automatically. 🎉

So now it's 💯 ready for primetime.

[natemoo-re]

Awesome, nice work!