fix: add mkFilePath and try to fix Windows build

src/Init/System/FilePath.lean

@@ -52,4 +52,8 @@ match fname.revPosOf pathSeparator with
 | some pos => { Substring . str := fname, startPos := 0, stopPos := pos }.toString
 
 end FilePath
+
+def mkFilePath (parts : List String) : String :=
+String.intercalate FilePath.pathSeparator.toString parts
+
 end System

tests/lean/run/parseCore.lean

@@ -2,7 +2,7 @@ import Init.Lean.Parser
 
 def test : IO Unit := do
 env ← Lean.mkEmptyEnvironment;
-Lean.Parser.parseFile env ("../../../src/Init/Core.lean");
+Lean.Parser.parseFile env (System.mkFilePath ["..", "..", "..", "src", "Init", "Core.lean"]);
 IO.println "done"
 
 #eval test