rename @sveltejs/kit/http to @sveltejs/kit/node

[Rich-Harris]

see #1271 (comment). This feels more appropriate — the http was always supposed to refer specifically to https://nodejs.org/api/http.html, but calling it node is more self-explanatory, and gives us a place to put other Node-specific helpers in future

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts