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/format.js

@@ -0,0 +1,30 @@
+// clang-format entry points
+const srcsToFmt = [
+  'modules/@angular/**/*.{js,ts}',
+  'modules/benchmarks/**/*.{js,ts}',
+  'modules/e2e_util/**/*.{js,ts}',
+  'modules/playground/**/*.{js,ts}',
+  'tools/**/*.{js,ts}',
+  '!tools/public_api_guard/**/*.d.ts',
+  './*.{js,ts}',
+  '!shims_for_IE.js',
+];
+
+module.exports = {
+  // Check source code for formatting errors (clang-format)
+  enforce: (gulp) => () => {
+    const format = require('gulp-clang-format');
+    const clangFormat = require('clang-format');
+    return gulp.src(srcsToFmt).pipe(
+        format.checkFormat('file', clangFormat, {verbose: true, fail: true}));
+  },
+
+  // Format the source code with clang-format (see .clang-format)
+  format: (gulp) => () => {
+    const format = require('gulp-clang-format');
+    const clangFormat = require('clang-format');
+    return gulp.src(srcsToFmt, {base: '.'})
+        .pipe(format.format('file', clangFormat))
+        .pipe(gulp.dest('.'));
+  }
+};