Fix scala-js/scala-js#3351: Use Buffer.alloc and Buffer.from instead of new Buffer().

The latter was deprecated a long time ago, and has started printing
noisy warnings to the console in Node.js v10.

There is still one deprecation inside jszip, but there is nothing
we can do about that one.

Author: sjrd

js-envs/src/main/scala/org/scalajs/jsenv/nodejs/AbstractNodeJSEnv.scala

@@ -172,7 +172,7 @@ abstract class AbstractNodeJSEnv(
              |  var recvCallback = null;
              |
              |  // Buffers received data
-             |  var inBuffer = new Buffer(0);
+             |  var inBuffer = Buffer.alloc(0);
              |
              |  function onData(data) {
              |    inBuffer = Buffer.concat([inBuffer, data]);
@@ -227,7 +227,7 @@ abstract class AbstractNodeJSEnv(
              |      if (socket === null) throw new Error("Com not open");
              |
              |      var len = msg.length;
-             |      var buf = new Buffer(4 + len * 2);
+             |      var buf = Buffer.allocUnsafe(4 + len * 2);
              |      buf.writeInt32BE(len, 0);
              |      for (var i = 0; i < len; ++i)
              |        buf.writeUInt16BE(msg.charCodeAt(i), 4 + i * 2);