This PR names the `repeat` syntax (`doRepeat`) and installs dedicated
elaborators for it in both the legacy and new do-elaborators. Both
currently expand to `for _ in Loop.mk do ...`, identical to the existing
fallback macro in `Init.While`.
The elaborators are dead code today because that fallback macro fires
first. A follow-up PR will drop the macro (after this PR's stage0 update
lands) and extend `elabDoRepeat` to choose between `Loop.mk` and a
well-founded `Repeat.mk` based on a `backward.do.while` option.
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