feat: add map_get function for cloning and returning values from a map

[makspll]

Summary

• Adds map_get binding
• Same signature as get but is a temporary stop gap for the lack of support for reflecting into hashmaps from get (due to missing functionality upstream in Bevy)
• The values are returned by value not by reference
• Adds some docs around the area

local Resource = world.get_type_by_name("TestResourceWithVariousFields")
local resource = world.get_resource(Resource)

assert(resource.string_map:map_get("foo") == "bar", "Expected bar, got " .. resource.string_map:map_get("foo"))

Came out from the discussion around #341