Add warning diagnostic for V2 API use

[microbit-matt-hillsdon]

Alternative to #33

I've tried to pull the logic into checker.ts which feels more suitable for a high-level check.

Test data (I think the key parts of this are now in the automated tests):

# Imports go at the top
from microbit import *
from microbit import run_every
import log
import neopixel
from power import deep_sleep
import power

power.off()

deep_sleep(10)

log.add({
  'temperature': temperature(),
  'sound': microphone.sound_level(),
  'light': display.read_light_level()
})

pin_logo.get_mode()
pin_speaker
Sound.GIGGLE

audio.SoundEffect()

pin0.CAPACITIVE

np = neopixel.NeoPixel(pin12, 8)
np.fill((0, 0, 233))
np.write()

[microbit-robert]

LGTM. Works well. One comment to remove an outdated comment.