[FEATURE REQUEST] Add option to run this as a builder

### :speech_balloon: Description
When using package to extract pubspec.yaml values into dart file called [pubspec_extract](https://pub.dev/packages/pubspec_extract) I have three choices. Either to install and run it globally, or to use it locally. Both should be possible with flutter_launcher_icons as well at the moment. But the most useful one, running it as a builder plugin (right before each build) is missing in this package. It might be useful to inspire yourself from their approach and make us able to use this way of running your script as well!

```yaml
builders:
flutter_launcher_icons: ^0.9.2
```

### :question: Platform
I want this for Android, but it should be platform-independent thing.

Start by studying how pubspec_extract exposes its builder approach and how flutter_launcher_icons currently supports global or local use. Define the builder configuration and integration needed so the package runs before each build in a platform-independent way, then verify the documented builders YAML usage.

Written by the indexing model from the issue text.