feat: add `screen.level` + `screen.update`

[dndrks]

this PR adds a few bits for folks used to norns scripting:

• screen.level is a monochromatic level/color helper, which just draws varying levels of grayscale
• screen.update is an alias for _seamstress.screen_refresh(), which aligns with expectations of the norns screen module

i know there are a lot of other screen module changes, but i feel like these are good initial stumbling blocks which can help ease the introduction. please lmk if you have any additional thoughts or q's!!

function init()
  width = 128
  height = 64
  screen.set_size(width, height)
end

function redraw()
  screen.clear()
  for i = 0, 15 do
    screen.move(i * (width / 16)+1, height/3)
    screen.level(15 - i)
    screen.text(i)
    screen.move_rel(0,10)
    screen.level(i)
    screen.text(i)
  end
  screen.update()
end