occurrences: Replace machinery-ref heuristics with JL's `synthetic_ref` #847

Job	Run time
Test with `JULIA_NUM_THREADS = 4,2`	8m 44s
Test with `JETLS_DEV_MODE = true`	13m 28s
Self diagnostics	6m 35s
Test with `JETLS_DEBUG_LOWERING = true`	10m 18s
Julia nightly - ubuntu-latest - x64	3m 10s
Julia 1.12 - ubuntu-latest - x64	8m 33s
Julia 1 - ubuntu-latest - x64	10m 37s
check_schemas / Check schemas are up to date	3m 28s
Julia 1 - macos-latest - x64	15m 32s
test_app / Test jetls check	5m 6s
test_app / Test jetls serve	4m 48s
test_app / Test jetls schema	5m 35s
	1h 35m 54s

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