WIP: Deprecate using variables declared in loops from closures

[tim-dlang]

Using variables declared in loops from a closure can have unexpected results and be unsafe (issue #19929). Fixing the problem would silently change the behaviour of code and result in more allocations, so deprecating the affected code first avoids surprises. A new implementation could later be introduced using a preview switch or editions.

Limitations:

• Loops manually created with goto are not detected.
• Problems involving lazy can be undetected, because of separate issues like #18669.
• Local functions, which don't escape, can result in false positives, because the compiler does not always know they don't escape. This can sometimes be fixed by marking delegate parameters as scope. This can also affect range based code.
• Loops with at most one iteration are still treated as loops and could be considered false positives.
• Closures, which are only used in the last iteration of a loop, still can result in the deprecation warning, which could be considered a false positive.
• There can also be false negatives, when a scope delegate escapes the loop, but not the function.

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dub run digger -- build "master + dmd#21936"