Multiple append() and prepend() in environment() object

append() and prepend() methods can now be called multiple times on the same varname. Earlier Meson versions would warn and only the last opperation was taking effect.

env = environment()

# MY_PATH will be '0:1:2:3'
env.set('MY_PATH', '1')
env.append('MY_PATH', '2')
env.append('MY_PATH', '3')
env.prepend('MY_PATH', '0')