Expose rawBody

[Rich-Harris]

This closes #831, making rawBody available to getContext, handle and endpoints.

It also derives request.body from request.rawBody inside Kit, rather than making adapters jump through hoops. This will make it easier to build lambda-based adapters, for example.

• update docs
• update adapter-begin (though i'm not sure what state this is in currently) will do this in a follow-up
• update adapter-cloudflare-workers
• update adapter-netlify
• update adapter-vercel
• add test of reading rawBody
• figure out API details. parse_body doesn't need to be exposed outside the package, but get_raw_body probably does, in some form. is @sveltejs/kit/http the right place for it? We typically defer to JS ecosystem conventions for public APIs, so it probably ought to be getRawBody

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