Add a `background-type` paint property

Nobody has claimed this yet.

## Motivation

We've heard accounts of it being confusing that a checkered background was shown in Studio when choosing a projection other than `mercator`:


We create that checkered background in Studio **but more broadly**: It's slightly weird (exacerbated by `globe`) that a `background` layer wraps around a projection itself and isn't strictly the background styling of the canvas.

## Proposal

An enum that let's us change the behavior of background to render in governance to its projection or the viewport.

```json
"background-type": {
"type": "enum",
"default": "map",
"values": {
"map": {
"doc": "Renders the background layer to the projected map"
},
"viewport": {
"doc": "Renders the background layer to the canvas"
}
}
}
```

Doing something would also allow us to apply a `background-pattern` (i.e creating a starry space backdrop).

---

**Mapbox GL JS version:** `2.7.0-alpha.1`

The issue names Studio, the background layer, projections, and Mapbox GL JS 2.7.0-alpha.1, but no source files or tests. Start by locating the background paint-property definition and the rendering paths for projected maps and the canvas; done means supporting the map and viewport enum values while preserving the current default behavior.

Written by the indexing model from the issue text.