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.