Add `async while` for repeating clock

[WingedSeal]

async while ($num == 1) {
  ...
} 1s;
async while ($num == 1) {
  ...
}
async while (true) {
  ...
}

[WingedSeal]

Added in https://github.com/WingedSeal/jmc/commit/605ce142b0a5985fe2d677cd0ef8e41fee255206

But timing is now required and cannot be implicitly left out