Fix the miscalculation of the start / end of the js directives lines.
Also small optimizations on usage of increment operators. Add a unit
test.
Fixes: QTCREATORBUG-29001
Change-Id: I38923f137dca5c0b89d474cd747a64ec11e62fd9
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
