Support pitch top-level interpolate expression

Motivation

I would like to disable terrain when the camera is pointing down so that the map doesn't deform, specially when showing a raster layer featuring baked-in text. I would like to smoothly transition it in and out with an expression using pitch.

Notice how the pink line is wobbly on the first picture. This is particularly obvious when moving the camera around.

Design Alternatives

There is obviously this rather tedious option:

map.on('pitch', () => {
    const pitch = map.getPitch();
    
    const newExaggeration = pitch > 30 ? (pitch / 30) : 1.0;

    map.setTerrain({
        'source': 'mapbox-dem',
        'exaggeration': newExaggeration
    });
});

Mock-Up

      terrain: {
        source: 'mapbox-dem',
        exaggeration: ['interpolate', ['linear'], ['pitch'], 0, 0, 20, 1],
      },

Concepts

This option would fit nicely with the existing top-level zoom interpolation logic

Start by locating the existing top-level zoom interpolation logic mentioned in the issue, then trace how terrain exaggeration is read and applied. Implement the requested pitch-based interpolation for terrain exaggeration and verify that terrain transitions as camera pitch changes without requiring a pitch event handler.