Fix HMR with full assetPrefix URLs

[kane-c]

We have a Next.js server running on a different host from the client-facing app. We set assetPrefix accordingly (e.g. http://localhost:3000) which works fine, except for HMR, which has the following error repeating in the console:

WebSocket connection to http://localhost:3000/_next/webpack-hmr failed:

With no further detail.

This works fine in Next.js 13.5 but breaks in 14.x.

This patch generates the correct URL for the HMR endpoint when assetPrefix is a full http/https URL.

[ijjk]

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