Unify the web Response binding into Fetch.Response

[reebalazs]

App Router route handlers return a web Response, but the bindings had no way to construct one — only the read side (FetchResponse, what fetch resolves to). Since it's one Web API type, this folds the construct side into the existing record module rather than adding a second type.

What

• Rename FetchResponse → Response, kept as a deprecated module FetchResponse = Response alias so existing consumers keep compiling.
• Add make(body, init) (new Response; body Nullable.t<string>, init = {status?, statusText?, headers?: Headers.t}) and the static redirect(url, status).
• Add a headers: Headers.t read field — response headers weren't readable at all before.
• fetch now returns Response.t.

Headers are typed Headers.t (consistent with NextServer.NextResponse.options), not a dict.

Breaking-change analysis

The record stays a record, so read access is unchanged (res.status, res.json()), and FetchResponse survives as an alias — so:

	Breaks?
Read access res.status / res.json()	No (record unchanged)
FetchResponse name / fetch return type	No (alias = same type)
Adding the headers field	Only breaks constructing a FetchResponse.t record literal

The one theoretical break (record-literal construction) has zero occurrences across rescript-next, njc, and vialab (checked: vialab consumes only Fetch.Headers + fetchJson, never FetchResponse).

Tested against the runtime (status + headers round-trip for make and redirect). No version bump (left for the release).