Move lit.dev tutorial element into playground-elements

The tutorial at https://lit.dev/tutorial/ uses playground-elements for the RHS, and a custom `` element for the LHS (https://github.com/lit/lit.dev/blob/main/packages/lit-dev-content/src/components/litdev-tutorial.ts). We should make this a bit more generic (i.e. don't hard-code the list of step filenames) and move it into this repo as ``.