build: modularize the gulp file to be easier to maintain (#14259)

This is a precursor to bringing in some of the gulp tasks from angular.io,
which could lead to the gulpfile becoming unwieldy.

tools/gulp-tasks/build.js

@@ -0,0 +1,6 @@
+
+module.exports = (gulp) => (done) => {
+  const path = require('path');
+  const childProcess = require('child_process');
+  childProcess.exec(path.join(__dirname, '../../build.sh'), done);
+};