This adds support for eslint's new flat config file feature.

Supporting both systems in one config object is a rather messy
situation, so we've opted here for multiple distinct configs instead.

Old style config files can continue using config objects:

```json
{
  extends: ['plugin:lit/recommended']
}
```

While new style config files can use the `flat/recommended` variety:

```json
[
  {
    "files": ["*.js"],
    ...configs['flat/recommended']
  }
]
```