build(docs-infra): add support for examples of type `elements` (#24840)

Examples using `@angular/elements` need to transpile to es2015 for
Custom Elements to work (on browsers that natively support them).

Alternatively, a polyfill would need to be loaded. For now, changing the
transpilation target to es2015 is the simplest solution.

PR Close #24840

aio/content/examples/elements/example-config.json

@@ -0,0 +1,3 @@
+{
+  "projectType": "elements"
+}

aio/tools/examples/example-boilerplate.js

@@ -50,13 +50,12 @@ const BOILERPLATE_PATHS = {
 // This maps the CLI files that exists in a parent folder
 const cliRelativePath = BOILERPLATE_PATHS.cli.map(file => `../cli/${file}`);
 
-BOILERPLATE_PATHS.i18n = [
+BOILERPLATE_PATHS.elements = [
   ...cliRelativePath,
-  'angular.json',
-  'package.json'
+  'tsconfig.json'
 ];
 
-BOILERPLATE_PATHS.universal = [
+BOILERPLATE_PATHS.i18n = [
   ...cliRelativePath,
   'angular.json',
   'package.json'
@@ -67,6 +66,12 @@ BOILERPLATE_PATHS.testing = [
   'angular.json'
 ];
 
+BOILERPLATE_PATHS.universal = [
+  ...cliRelativePath,
+  'angular.json',
+  'package.json'
+];
+
 const EXAMPLE_CONFIG_FILENAME = 'example-config.json';
 
 class ExampleBoilerPlate {

aio/tools/examples/shared/boilerplate/elements/tsconfig.json

@@ -0,0 +1,20 @@
+{
+  "compileOnSave": false,
+  "compilerOptions": {
+    "outDir": "./dist/out-tsc",
+    "sourceMap": true,
+    "declaration": false,
+    "moduleResolution": "node",
+    "emitDecoratorMetadata": true,
+    "experimentalDecorators": true,
+    "target": "es2015",  // Custom Elements require ES2015 classes (or polyfill).
+    "skipLibCheck": true,
+    "typeRoots": [
+      "node_modules/@types"
+    ],
+    "lib": [
+      "es2017",
+      "dom"
+    ]
+  }
+}