rename app.render to server.respond

[Rich-Harris]

Closes #4030 — class App becomes class Server

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@sveltejs/adapter-cloudflare-workers	Patch
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[benmccann]

CC'ing the community adapter authors: @jthegedus @pluvial @geoffrich @MacFJA @HalfdanJ @ryanbethel

As part of putting up our new types documention we renamed the App.render to Server.respond to avoid conflicting with the App namespace`, so a small update will be needed to any adapter