[JuliaLowering] Restore partial Julia 1.12 compatibility (#62367)
Recent JuliaLowering changes unconditionally referenced runtime features
that are unavailable on Julia 1.12. This prevented JuliaLowering and
JuliaSyntax from loading or lowering code on that release (esp. in
JETLS).
Guard `Core.MacroSource`, `Core.TypeEqOf`, and
`jl_lowering_world` behind availability checks, falling back to
`Core.Typeof` and direct invocation where needed.
The `include_string` precompile workload remains restricted to newer
runtimes because its evaluation path is incompatible with Julia 1.12.
Keep the preceding lowering-only workloads enabled there: they do not
evaluate lowered code, still exercise the basic lowering pipeline, and
remain usable on Julia 1.12.
