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/lint.js

@@ -0,0 +1,29 @@
+// Check the coding standards and programming errors
+module.exports = (gulp) => () => {
+  const tslint = require('gulp-tslint');
+  // Built-in rules are at
+  // https://palantir.github.io/tslint/rules/
+  const tslintConfig = require('../../tslint.json');
+  return gulp
+      .src([
+        // todo(vicb): add .js files when supported
+        // see https://github.com/palantir/tslint/pull/1515
+        './modules/**/*.ts',
+        './tools/**/*.ts',
+        './*.ts',
+
+        // Ignore TypeScript mocks because it's not managed by us
+        '!./tools/@angular/tsc-wrapped/test/typescript.mocks.ts',
+
+        // Ignore generated files due to lack of copyright header
+        // todo(alfaproject): make generated files lintable
+        '!**/*.d.ts',
+        '!**/*.ngfactory.ts',
+      ])
+      .pipe(tslint({
+        tslint: require('tslint').default,
+        configuration: tslintConfig,
+        formatter: 'prose',
+      }))
+      .pipe(tslint.report({emitError: true}));
+};