void: Add checkpoints to void runner levels

[wjt]

In the Loom Lounge yesterday, Stephen noticed that when I was caught in
void_runner, there was no Knitwitch when I respawned.

I don't think we strictly need to have Knitwitches at the start of every
level where you might be defeated. If you are caught by the very first
guard in the musician quest stealth level, you just respawn at the start
of the level, no witch.

However, perhaps it's reassuring to have one at the start of
void_runner. Having done that, maybe we can also put one after the
tricky three-needle crossing in void_grappling_round_2, and then also at
the start of that scene for good measure.

There is no good place (other than the very start) to put one in
void_grappling round 1.

[github-actions]

Play this branch at https://play.threadbare.game/branches/endlessm/wjt/void-add-checkpoints-to-void-runner-levels/.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[wjt]

Draft because I'm not sure this is necessary.

[manuq]

I played through it and it certainly feels better! I remember we discussed about it before, for persisting the Void tiles. Having played it, I don't think that is necessary.

[manuq]

I particularly like the nitwitcher as it appears after the pin.

[wjt]

Unlike the situation in #1208 these are all placed before (not after) sections where the void chases you. So there is no need to persist tiles that the chasing-void has eaten!