feat(ng-add): add x-prompts to install schematics (#13058)

* Recently the Angular CLI project introduced prompt support for schematics. Even though they seem to not run when using `7.0.0-beta.2` of the CLI, we should already ship the simple prompts for the `ng add` command.
* Renames the `/schematics/install` folder to `/schematics/ng-add`. (this makes it consistent with the actual schematic name and other packages that ship the `ng-add` schematic (e.g. `@angular/elements`)

src/lib/schematics/collection.json

@@ -5,8 +5,8 @@
     // Adds Angular Material to an application without changing any templates
     "ng-add": {
       "description": "Adds Angular Material to the application without affecting any templates",
-      "factory": "./install",
-      "schema": "./install/schema.json",
+      "factory": "./ng-add/index",
+      "schema": "./ng-add/schema.json",
       "aliases": ["material-shell", "install"]
     },
     // Create a dashboard component

src/lib/schematics/ng-add/index.spec.ts

@@ -9,7 +9,7 @@ import {collectionPath, createTestApp} from '../test-setup/test-app';
 import {getWorkspace, WorkspaceProject} from '@schematics/angular/utility/config';
 import {normalize} from '@angular-devkit/core';
 
-describe('material-install-schematic', () => {
+describe('ng-add schematic', () => {
   let runner: SchematicTestRunner;
   let appTree: Tree;
 

src/lib/schematics/ng-add/schema.json

@@ -1,7 +1,7 @@
 {
   "$schema": "http://json-schema.org/schema",
-  "id": "SchematicsMaterialInstall",
-  "title": "Material Install Options Schema",
+  "id": "angular-material-ng-add",
+  "title": "Angular Material ng-add schematic",
   "type": "object",
   "properties": {
     "project": {
@@ -19,12 +19,14 @@
     "theme": {
       "enum": ["indigo-pink", "deeppurple-amber", "pink-bluegrey", "purple-green", "custom"],
       "default": "indigo-pink",
-      "description": "The theme to apply"
+      "description": "The theme to apply",
+      "x-prompt": "Enter a prebuilt theme name, or \"custom\" for a custom theme:"
     },
     "gestures": {
       "type": "boolean",
       "default": true,
-      "description": "Whether gesture support should be set up or not."
+      "description": "Whether gesture support should be set up or not.",
+      "x-prompt": "Set up HammerJS for gesture recognition?"
     }
   },
   "required": []