Unable to perform polygon boolean operation on a list of polygons simultaneously

Godot version: 3.2, 4.0

Pertains to #28987.

Issue description: Currently it's not as easy to perform polygon boolean operations with Geometry methods like merge_polygons_2d on a list of polygons. One has to use these methods in multiple iterations, for instance, to merge a list of polygons that may or may not be adjacent to each other, see this use case.

Steps to reproduce:

var polygons = [] # assume they all can be merged into one polygon
var poly_a = polygons[0]

for idx in range(1, polygons.size()):
    var poly_b = polygons[idx]
    var solution = Geometry.merge_polygons_2d(poly_a, poly_b)
    assert(solution.size() == 1) # might fail as polygons may be far apart
    poly_a = solution[0]

@Dr4kzor, @avencherus