libexpr: Warn on incorrect string escapes

In Nixpkgs, there are several strings like "\d\.\d" which attempt to be
a regex but are just literally "d.d". The escaping rules are silly and
we should warn our users about that.

Co-authored-by: Commentator2.0 <lix@crystal-cavern.systems>
Change-Id: I779b0757358fc9adc34dc140e1670b83abc93b67

tests/functional2/lang/string/test.toml

@@ -0,0 +1,9 @@
+[[test]]
+runner = "eval-okay"
+matrix = true
+flags = ["--extra-deprecated-features", "broken-string-escape"]
+in = ["in.nix", "in-backslash-newline.nix"]
+
+[[test]]
+runner = "eval-okay"
+in = "in-escapes.nix"