Experimental: Adds declarative properties to the Blueprint schema (#788)

## What is this PR doing?

First stab at
https://github.com/WordPress/wordpress-playground/issues/787

This PR proposes a first few declarative Blueprint properties:

```js
{
	"landingPage": "/wp-admin/post-new.php",
	"plugins": [ 
		"gutenberg", 
		"classic-editor",
		"https://mysite.com/plugin.zip",
		{ "resource": "url", "https://mysite.com/plugin.zip" }
	],
	"login": true,
	"siteOptions": {
		"blogname": "My site!"
	}
}
```

Under the hood, all these declarative properties are translated into
`steps` and prepended to the list of Blueprint steps. Therefore, the
imperative `steps` API is still supported and can be mixed with the
declarative syntax.

These new options are all marked as deprecated from the start to
discourage their use. Once they're stable, I'll remove the deprecation
notice.

## Testing instructions

* Pull this branch
* Paste the below Blueprint at http://localhost:5400/website-server/#
<-- here
* Confirm it worked as expected

```js

```js
{
	"landingPage": "/wp-admin/post-new.php",
	"plugins": [ 
		"gutenberg", 
		"classic-editor"
	],
	"login": true,
	"siteOptions": {
		"blogname": "My site!"
	}
}
```



cc @dmsnell @seanmorris