[vector_graphics] Add an example to vector_graphics_compiler

Currently there's no example, so the corresponding tab doesn't show up on pub.dev; this may make it harder for people who end up at that package before `vector_graphics` for whatever reason to understand how to use it (and also causes a pana score deduction).

We could either make a full example app, showing how to use it (along the lines of https://docs.flutter.dev/ui/assets/asset-transformation), or we could add a transformer to the `vector_graphics` example and put an `example/example.md` file in this package that links to the `vector_graphics` example (and maybe that docs page as well).