Add samples index (#359)

* add samples_index to web/ directory

Co-authored-by: Thea Flowers <theaflowers@google.com>

* add pub_get.dart script

* build sample index in peanut post build

* re-generate sample index with web demos

* print more details in peanut_post_build.dart

* add images for demos

* run generator

* update README

* add animations and provider shopper as symlinks

* add links to symlinked web demos

* use relative paths

* update cookbook images, urls, and description CSS

* use relative URL for navbar link

* unstage HTML files

* .gitignore generated HTML files

* add margin to toolbar

* rename escape functions

* add and update copyright headers

Co-authored-by: Thea Flowers <theaflowers@google.com>

web/samples_index/lib/src/util.dart

@@ -0,0 +1,33 @@
+// Copyright 2020 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file
+
+import 'dart:convert';
+
+String indent(String content, int spaces) =>
+    LineSplitter.split(content).join('\n' + ' ' * spaces);
+
+String kebabCase(String input) => _fixCase(input, '-');
+
+String snakeCase(String input) => _fixCase(input, '_');
+
+final _upperCase = RegExp('[A-Z]');
+
+String pascalCase(String input) {
+  if (input.isEmpty) {
+    return '';
+  }
+
+  return input[0].toUpperCase() + input.substring(1);
+}
+
+String _fixCase(String input, String separator) =>
+    input.replaceAllMapped(_upperCase, (match) {
+      var lower = match.group(0).toLowerCase();
+
+      if (match.start > 0) {
+        lower = '$separator$lower';
+      }
+
+      return lower;
+    });