LibWeb: Add NavigateEvent.sourceElement #10124

Job	Run time
Lagom (Linux, ubuntu-24.04, NO_FUZZ, GNU, false) / CI	1h 15m 6s
Lagom (macOS, macos-15, NO_FUZZ, Clang, false) / CI	1h 19m 54s
Lagom (Linux, ubuntu-24.04, FUZZ, Clang, false) / CI	48m 31s
Lagom (Linux, ubuntu-24.04, NO_FUZZ, Clang, true) / CI	1h 9m 5s
	4h 32m 36s

Provide feedback