Add tilemap files support to avoid non-existing tiles requesting

## Motivation

I have an issue with a vector tile source that contains some data for some country:

Vector tiles server has tiles for country's capital at 12 zoom level.
Other region's tiles exist only for <= 10 zoom level.

If I set `maxzoom: 12` for my source, map works well, but only for capital region.
I see blank map for non-capital regions.

## Design Alternatives

As a workaround I can add 2 sources (specifying `maxzoom: 10` for the first one and `minzoom: 10, maxzoom: 12, bounds: [capital-bounds]` for the second ) and duplicate all layers for them.

Unfortunately it generates a new error - layers overlay.

## Design

I want mapbox to add new optional property for sources - tilemap that will specify an URL to load it.
It could be cumbersome, but it will solve the root issue

https://developers.arcgis.com/rest/services-reference/vector-tilemap.htm

Start by reviewing the vector tile source-loading path and the ArcGIS vector tilemap specification linked in the issue; no source files or tests are identified. Define the tilemap source property and verify that regions with missing higher-zoom tiles fall back without requiring duplicated sources or causing layer overlays.

Written by the indexing model from the issue text.