fix: use magic-string to create sourcemaps

[danielroe]

related: #1

This uses magic-string to track and create sourcemaps for modified SFCs. (It defaults to disabled unless sourcemap: true is passed to the unplugin.) Bear in mind this can produce higher memory usage, but it is generally worth it.

In addition there are a couple of performance improvements baked in.

1. In general we have rollup performance benefits by not returning anything from a transform hook if nothing has changed.
2. We also skip injecting the imports if the component hasn't been added - this should save a bit of time.