Dartdoc snippet extension to inject full featured code snippets in to API docs. (#23281)

This creates a custom dartdoc tool that will generate snippet blocks in our API docs that allow the user to copy easily to the clipboard, and will also embed the snippet code into a template to show it in a larger context with an app.

This PR adds the snippet tool, a template, and a couple of HTML skeleton files, one for snippets that are designed to be in an application setting, and one where it simply puts a nice container around existing snippets, making them easier to copy to the clipboard.

dev/bots/analyze.dart

diff --git a/dev/bots/analyze.dart b/dev/bots/analyze.dart
index 1b911ef..7b25c5a 100644
--- a/dev/bots/analyze.dart
+++ b/dev/bots/analyze.dart
@@ -266,6 +266,7 @@
         if (path.split(file.path).contains('test_driver') ||
             name.startsWith('dev/missing_dependency_tests/') ||
             name.startsWith('dev/automated_tests/') ||
+            name.startsWith('dev/snippets/') ||
             name.startsWith('packages/flutter/test/engine/') ||
             name.startsWith('examples/layers/test/smoketests/raw/') ||
             name.startsWith('examples/layers/test/smoketests/rendering/') ||