Dynamically set filtering based on another layer

Nobody has claimed this yet.

## Motivation
I have two layers, one is the polygons with yellow outlines and the other one is the purple dots. I want to highlight the yellow outlined polygons by filling them blue when there's one or multiple purple dots on them, but I couldn't find a good way to do so.

## Design Alternatives

The closest thing I could find is "within" (https://docs.mapbox.com/mapbox-gl-js/style-spec/expressions/#within) that I can use in expressions, but it only returns true when the features is **fully** contained inside the input geometry.

## Design

I would suggest adding another expression called "touch", which is similar to "within", but instead of only "returns true if the evaluated feature is fully contained inside a boundary of the input geometry", we can make it "returns true if the evaluated feature touches the boundary of the input geometry"

The issue names no implementation files or tests. Start by reviewing the style-spec documentation for the existing `within` expression and how filtering is evaluated across layers, then clarify the intended boundary-intersection semantics. Done means the expression behavior is agreed and can identify polygon features touched by points from another layer.

Written by the indexing model from the issue text.