Split out CmdRepl and editorFor

The REPL itself and the `nix repl` CLI are conceptually different
things, and thus deserve to be in different files.

src/libcmd/editor-for.hh

@@ -0,0 +1,11 @@
+#pragma once
+
+#include "types.hh"
+
+namespace nix {
+
+/* Helper function to generate args that invoke $EDITOR on
+   filename:lineno. */
+Strings editorFor(const Path & file, uint32_t line);
+
+}