Add more npm commands

[123mpozzi]

Since we are already using npm for husky, we could leverage it to add more useful commands.
The commands can be run via npm run <command> everywhere in the project folder.

New Commands

• lint to run linting for each platform
• build to start the build runner
• build:clean to start the build runner telling it to remove conflicting output
• install to install dependencies
• install:clean to install dependencies after removing build folders
• start to run the example app
• test:integration to run example integration tests
• docs to generate docs
• fix:caching to try fixing dart/flutter caching issues (eg. new .env values are not being updated correctly in example/lib/env/env.g.dart)