[now-next] Implement new handles for custom routes (#3892)

This implements the new handles from https://github.com/zeit/now/pull/3876 to allow us to ensure the proper order for `rewrites`, `redirects`, and `headers` in Next.js. I also added in the tests from the Next.js [custom-routes test suite](https://github.com/zeit/next.js/tree/canary/test/integration/custom-routes) to ensure we're matching behavior. 

To help keep track of what each probe is testing I added support for parsing the `now.json` files in `testDeployment` as [JSON5](https://www.npmjs.com/package/json5) to allow adding comments before each probe. If this is undesired I can remove this specific change even though it makes managing the fixture tests much easier

test/lib/deployment/test-deployment.js

@@ -1,6 +1,7 @@
 const assert = require('assert');
 const bufferReplace = require('buffer-replace');
 const fs = require('fs');
+const json5 = require('json5');
 const glob = require('util').promisify(require('glob'));
 const path = require('path');
 const { spawn } = require('child_process');
@@ -52,7 +53,8 @@ async function testDeployment(
     );
   }
 
-  const nowJson = JSON.parse(bodies['now.json']);
+  // we use json5 to allow comments for probes
+  const nowJson = json5.parse(bodies['now.json']);
 
   if (process.env.NOW_BUILDER_DEBUG) {
     if (!nowJson.build) {