Interpolated heatmap with contour line in VegaLite.jl

Looking to generate a continous heatmap of data, with a contour line, something line the following plot:

Example data if someone wants to play with it: (note that there is a row for column names, and a column for the y axis.

Today I tried but without success.
I think you should follow this (Vega) example:

The contour line could be a single isoline at 1.0

VegaLite does not yet support contour plots