Generate `baler-preload-map.json`

@DrewML is already working on this.

Since Aug 20, 2019.

For the [`Graph Preloading`](https://github.com/DrewML/baler/blob/58e58744d9e7cf79e35c7b8cac9123cce917a598/docs/HOW_IT_WORKS.md#graph-preloading) functionality, it's necessary for `baler` to spit out a mapping of JavaScript dependencies needed per `.phtml` file.

## Requirements
1. Mapping file should be named `baler-preload-map.json`, and should be written to the root of a locale dir, as a sibling of `requirejs-baler-config.js`
2. A [JSON Schema](https://json-schema.org/) should be provided for `Magento_Baler` to use for validation
3. Key/Value should be
- Key: Path from store root to a `.phtml` file
- Value: Array of relative paths to JavaScript files from the require basedir

## Example

```json
{
"vendor/magento/module-example/view/frontend/templates/some-template.phtml": ["path/to/file/1.js", "path/to/file/2.js"],
"vendor/thirdparty/module-bar/view/frontend/templates/another-template.phtml": ["another/file.js", "yet/another/file/2.js"]
}
```