feat(net): vim.net.request(), :edit [url] #34140

Problem:
Nvim depends on netrw to download/request URL contents.

Solution:
- Add `vim.net.request()` as a thin curl wrapper:
  - Basic GET with --silent, --show-error, --fail, --location, --retry
  - Optional `opts.outpath` to save to a file
  - Operates asynchronously. Pass an `on_response` handler to get the result.
- Add integ tests (requires NVIM_TEST_INTEG to be set) to test success
  and 404 failure.
- Health check for missing `curl`.
- Handle `:edit https://…` using `vim.net.request()`.

API Usage:
1. Asynchronous request:

    vim.net.request('https://httpbingo.org/get', { retry = 2 }, function(err, response)
      if err then
        print('Fetch failed:', err)
      else
        print('Got body of length:', #response.body)
      end
    end)

2. Download to file:

    vim.net.request('https://httpbingo.org/get', { outpath = 'out_async.txt' }, function(err)
      if err then print('Error:', err) end
    end)

3. Remote :edit integration (in runtime/plugin/net.lua) fetches into buffer:

    :edit https://httpbingo.org/get

runtime/lua/vim/_editor.lua

@@ -41,6 +41,7 @@ for k, v in pairs({
   snippet = true,
   pack = true,
   _watch = true,
+  net = true,
 }) do
   vim._submodules[k] = v
 end