[vector_graphics] Add an example to vector_graphics_compiler

[stuartmorgan-g]

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).

[sheershtehri7]

I've submitted a PR to flutter/packages that implements the second approach suggested here:

flutter/packages#11313

What the PR does:

• Adds example/example.md to vector_graphics_compiler so pub.dev shows an Example tab and the pana score deduction is resolved
• Extends the vector_graphics example app with a build-time transformer demo using AssetBytesLoader, alongside the existing NetworkSvgLoader runtime demo