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/eval-okay-escapes.err.exp

@@ -0,0 +1,26 @@
+warning: \a is an ill-defined escape. You can drop the \ and simply write a instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:1:3:
+              1| "\a\n\{\$\"\'\r\v\\\t
+               |   ^
+              2| \f"
+warning: \{ is an ill-defined escape. You can drop the \ and simply write { instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:1:7:
+              1| "\a\n\{\$\"\'\r\v\\\t
+               |       ^
+              2| \f"
+warning: \' is an ill-defined escape. You can drop the \ and simply write ' instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:1:13:
+              1| "\a\n\{\$\"\'\r\v\\\t
+               |             ^
+              2| \f"
+warning: \v is an ill-defined escape. You can drop the \ and simply write v instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:1:17:
+              1| "\a\n\{\$\"\'\r\v\\\t
+               |                 ^
+              2| \f"
+warning: \f is an ill-defined escape. You can drop the \ and simply write f instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:2:2:
+              1| "\a\n\{\$\"\'\r\v\\\t
+              2| \f"
+               |  ^
+              3|

tests/functional2/lang/string/eval-okay-escapes.out.exp

@@ -0,0 +1 @@
+"a\n{$\"'\rv\\\t\nf"

tests/functional2/lang/string/in-escapes.nix

@@ -0,0 +1,2 @@
+"\a\n\{\$\"\'\r\v\\\t
+\f"

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"