Add hook for testing JuliaLowering in core

Based on JuliaSyntax code.  Intended to help with JETLS experimentation, which
can't currently get info from e.g. Base, which is lowered by flisp

Author: mlechu

src/JuliaLowering.jl

@@ -33,6 +33,8 @@ _include("syntax_macros.jl")
 
 _include("eval.jl")
 
+_include("hooks.jl")
+
 function __init__()
     _register_kinds()
 end

src/hooks.jl

@@ -0,0 +1,20 @@
+const _has_v1_13_hooks = isdefined(Core, :_setlowerer!)
+
+function core_lowerer_hook(st::SyntaxTree, mod::Module, file, line)
+    to_lowered_expr(mod, lower(mod, st))
+end
+
+"""
+Use Julialowering in Core
+"""
+function activate!(enable=true)
+    if !_has_v1_13_hooks
+        error("Cannot use JuliaLowering in $VERSION < 1.13")
+    end
+
+    if enable
+        Core._setlowerer!(core_lowerer_hook)
+    else
+        Core._setlowerer!(nothing)
+    end
+end