Start devServer dual-stack by default

[LouisMT]

The default host for Webpack's devServer is currently 0.0.0.0. So we can reach our project on http://localhost:3000, but only over IPv4!

For browsers this isn't really a problem, as they fall back to IPv4 automatically. But other clients usually don't have such a fallback, and as more operating systems/frameworks will default to IPv6, this will cause problems.

By using :: instead of 0.0.0.0, the server will be available on both IPv4 and IPv6 by default.

I've tested this change locally, and the Node.js docs mention that dual-stack support is the default behaviour when using ::. But more testing might be needed.

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A little more context: I ran into this issue because I'm using setupProxy to access my GraphQL API through the devServer. So I ran graphql-codegen against http://localhost:3000/graphql, and it tries to connect using IPv6.

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