fix(js): give a dead region the reason of the terminator that starts it

[DaisukeYoda]

Ports pyscn 175de1f (pyscn#471), which jscan/SYNC.md held back until #172 was fixed.

A dead block took its reason from the last statement of a predecessor or from its own label. A block further down a dead region, such as the body of an if, a loop or a catch after a return, matched neither, so it was reported as an unreachable_branch warning. Findings of different reasons are not merged, so one dead region came out as several findings.

The detector now walks forward from each block the CFG builder created after a return, throw, break or continue. Every unreachable block on that walk takes that terminator's reason and critical severity, so the region merges into one finding.

A closing } is not a CFG statement, so a region whose nested if or loop ends before more dead code is still split at that line. jscan/SYNC.md records this gap. On imbue-ai/latchkey and sindresorhus/got the findings do not change.