commands/new: Improve theme creation

- Update the skeleton structure to match the new template system.
- Add a --format flag to the `hugo new theme` command to control the
  format of the site configuration and default archetype files.
- Remove theme.toml. This file's presence can be confusing for new
  users, and the README in the themes repository already has an example.
- Remove the LICENSE and README files from the skeleton. These files
  are not needed for a theme to work, and they can be added later by
  the user if desired.

Closes #13489
Closes #13544

create/skeletons/theme/layouts/home.html

@@ -0,0 +1,7 @@
+{{ define "main" }}
+  {{ .Content }}
+  {{ range site.RegularPages }}
+    <h2><a href="{{ .RelPermalink }}">{{ .LinkTitle }}</a></h2>
+    {{ .Summary }}
+  {{ end }}
+{{ end }}