feat: add a bunch of new Nebular demos (#1911)

src/app/pages/extra-components/tree/tree.component.ts

@@ -0,0 +1,33 @@
+import { Component } from '@angular/core';
+
+@Component({
+  selector: 'ngx-tree',
+  templateUrl: './tree.component.html',
+  styleUrls: ['./tree.component.scss'],
+})
+export class TreeComponent {
+
+  nodes = [{
+    name: 'Programming languages by programming paradigm',
+    children: [{
+      name: 'Object-oriented programming',
+      children: [{
+        name: 'Java',
+      }, {
+        name: 'C++',
+      }, {
+        name: 'C#',
+      }],
+    }, {
+      name: 'Prototype-based programming',
+      children: [{
+        name: 'JavaScript',
+      }, {
+        name: 'CoffeeScript',
+      }, {
+        name: 'Lua',
+      }],
+    }],
+  }];
+
+}