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-indented/depr-warning-escapes.err.exp

@@ -0,0 +1,42 @@
+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:2:4:
+              1| ''
+              2| ''\a''\n
+               |    ^
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+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:3:4:
+              2| ''\a''\n
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+               |    ^
+              4| ''\'''${"x"}
+warning: ''\{ is an ill-defined escape. You can drop the ''\ and simply write { instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:3:8:
+              2| ''\a''\n
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+               |        ^
+              4| ''\'''${"x"}
+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:3:12:
+              2| ''\a''\n
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+               |            ^
+              4| ''\'''${"x"}
+warning: ''\" is an ill-defined escape. You can drop the ''\ and simply write " instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:3:16:
+              2| ''\a''\n
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+               |                ^
+              4| ''\'''${"x"}
+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:3:28:
+              2| ''\a''\n
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+               |                            ^
+              4| ''\'''${"x"}
+warning: ''\\ is an ill-defined escape. You can drop the ''\ and simply write \ instead. Use --extra-deprecated-features broken-string-escape to silence this warning.
+         at /pwd/in.nix:3:32:
+              2| ''\a''\n
+              3| ''\f''\{''\$''\"''\'''\r''\v''\\''\t
+               |                                ^
+              4| ''\'''${"x"}

tests/functional2/lang/string-indented/depr-warning-escapes.out.exp

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

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

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

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

@@ -0,0 +1,5 @@
+''
+''\a''\n
+''\f''\{''\$''\"''\'''\r''\v''\\''\t
+''\'''${"x"}
+''

tests/functional2/lang/string-indented/test.toml

@@ -1,12 +1,13 @@
 [[test]]
 runner = "eval-okay"
 matrix = true
-flags = ["--extra-deprecated-features", "broken-string-indentation"]
+flags = ["--extra-deprecated-features", "broken-string-indentation broken-string-escape"]
 
 [[test]]
 name = "depr-warning"
 runner = "eval-okay"
-in = "in.nix"
+matrix = true
+in = ["in.nix", "in-escapes.nix"]
 
 [[test]]
 runner = "parse-okay"