evaluator: Add builtin sleep

[juliaogris]

Add the builtin sleep function: sleep 1 sleeps for one second,
sleep 0.001 sleeps for one milli second. Now that we have wasm
hoisted on the main() go function we can just map to Go's sleep
function which panic'ed before.

[github-actions]

firebase-deployment: https://evy-lang--66-r6m7pvp5.web.app (e359817)

[camh-]

for i := range 10 90
  color "white"
  move 0 0
  rect 100 100
  color "red"
  move i 10
  circle 10
  sleep 0.01
end

for i := range 10 90
  color "white"
  move 0 0
  rect 100 100
  color "red"
  move 90 i
  circle 10
  sleep 0.01
end

for i := range 90 10 -1
  color "white"
  move 0 0
  rect 100 100
  color "red"
  move i 90
  circle 10
  sleep 0.01
end

for i := range 90 10 -1
  color "white"
  move 0 0
  rect 100 100
  color "red"
  move 10 i
  circle 10
  sleep 0.01
end

[camh-]

🥦 LGTM