New Source types in v3.5 don't support type narrowing via the type property

[ezzatron]

Related to #13203

mapbox-gl-js version: v3.5.1

browser: N/A

Steps to Trigger Behavior

1. Write code that checks the type property of a Source to narrow its type.
2. Try to use the narrowed type source as the expected type.
3. Source type is not narrowed.

Link to Demonstration

https://www.typescriptlang.org/play/?#code/JYWwDg9gTgLgBAbzgcQKYQFIGUDyA5LCAVygGNUAaOAWQEMwBfOAMyghDgCIR6AjCAB4BaAOYAbTgG4AUNNIQAdgGd4PMHAC8cBagDuNegAoE8hTFrAdUAFxwAJhFJEQqMwDoRqGAFExqF2YAQgCeAJJ2hpzAdpwAlACEDLEypipwSsRkqJpwah5ehCTkkdFxMtJ+8J6YuASZ5LZoNfiFWeXAzHCGGUWoAPxuMMFg2RpjXNUAVhkKcYjScItw1dgt9aPp6zIMQA

Repeated here:

import { GeoJSONSource, Map} from "mapbox-gl";

const map = new Map({container: document.getElementById("id")!});
const source = map.getSource("id");

let geoJSONSource: GeoJSONSource;

if (source?.type === "geojson") {
    geoJSONSource = source; // <-- this should not be a type error, but it is
}

Expected Behavior

You can narrow a Source type to a GeoJSONSource type by using source.type === "geojson".

Actual Behavior

Type 'GeoJSONSource | VideoSource | ImageSource | CanvasSource' is not assignable to type 'GeoJSONSource'.

[stepankuzmin]

Thanks for creating an issue, @ezzatron. We'll address this in the upcoming patch release.