fix(pace-caret): keep going when words are not generated yet (@boergeson)

[boergeson]

Description

With plus_one, plus_two, plus_three etc only a couple of words exist ahead of the current one. When the pace caret got past the last generated word it threw, settings got set to null and the caret was gone for the rest of the test, even after you catched up and passed it.

Now when the pace caret runs out of words it hides and keeps counting the steps it could not take. Once more words are there it jumps ahead by that amount and carries on. Timing is untouched so the position stays exact. Checked with a 60 wpm custom pace on plus one: paused 3s after the first letter, then typed past it, the caret sat at char 22, 32 and 52 at 4.42s, 6.42s and 10.43s (expected 22.1, 32.1, 52.1).

Also added a unit test for pace-caret that covers running past the generated words and coming back.

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Closes #7527