build: add source maps into NPM distribution (#15159)

Previous to 2.x there were some source maps distrubted, but they didn't go
all the way back to the TypeScript sources and they weren't available for
all JavaScript distrubted to NPM.

With this change source maps will be available for FESM distributions as
well as UMD and will go all the way back to TypeScript sources.

Fixes #15184

scripts/build/map_sources.js

@@ -0,0 +1,13 @@
+
+var path = require('path');
+var sorcery = require('sorcery');
+var yargs = require('yargs');
+
+var argv = require('yargs')
+  .alias('f', 'file')
+  .argv;
+
+
+sorcery.load(argv.file).then(function(chain) {
+  chain.write();
+});