chore(build): Build a umd bundle of rxjs-compat

This introduces a build step for creating a umd bundle (and minified umd bundle)  of rxjs-compat.

This involved creating a new tsconfig for esm5 for rollup (no helpers), a new rollup build script for producing the bundle, and a closure build script for producing the minified bundle. Each was derived from their rxjs counterpart.

This also creates a `umd.ts` entrypoint in `rxjs-compat` which exposes `testing`, `operators`, etc. just like the rxjs umd entry.

Tested by using this bundle in place of the rxjs 5 bundle in [Nuclide](https://github.com/facebook/nuclide/)

Tested the performance impact of the unminified bundle by loading it in <=20ms with `for i in {1..5}; do node -p "s=Date.now();require('./dist-compat/global/rxjs-compat.umd');Date.now()-s"; done`. The commonjs entrypoint to `rxjs-compat` loaded in 130ms by comparison. Note this improvement over the [rxjs 5 bundle](#2064) which loads in 26ms -- I verified I could reproduce nearly @zertosh's exact values on the hardware I tested 6 on.

cc @zertosh @matthewwithanm @captbaritone

compat/umd.ts

@@ -0,0 +1,22 @@
+/*
+  NOTE: This is the global export file for rxjs v6 and higher.
+ */
+
+/* rxjs */
+export * from './Rx';
+
+/* rxjs.operators */
+import * as _operators from 'rxjs/operators';
+export const operators = _operators;
+
+/* rxjs.testing */
+import * as _testing from 'rxjs/testing';
+export const testing = _testing;
+
+/* rxjs.ajax */
+import * as _ajax from 'rxjs/ajax';
+export const ajax = _ajax;
+
+/* rxjs.webSocket */
+import * as _webSocket from 'rxjs/webSocket';
+export const webSocket = _webSocket;