Add/local dev environment

[gaambo]

I plan to suggest two PRs for the variations filter and had to set up a local dev environment to make changes. Therefore, I thought I might as suggest it as an PR for you and other contributors.

For convenience, I've added an easier local development environment using @wordpress/env.
This allows for simpler setup of local dev environment and also ensures all contributors have the same local dev setup. More information about that package here: https://developer.wordpress.org/block-editor/reference-guides/packages/packages-env/
I've also added npm scripts to make it easier to use the wp-env version installed in the package. The naming of these scripts is arbitrary.

I've also fixed the node/npm version used in this plugin. I guessed it was initially v14 (because all versions above try to migrate the package-lock.json). I then updated it to v16 to be on par with Gutenberg. This makes it easier for developers to use the correct node/npm version (using nvm).

Testing:

1. Make sure you have gut docker setup (see @wordpress/env prerequisites)
2. Run nvm use if you have multiple node versions installed
3. Run npm install
4. Run npm run dev:start to startup local dev environment
5. Visit WordPress (probably on localhost:8888) and ensure the plugin is installed, active and usable.

Nothing about the plugin functionality or the build process changes/should change.