libexpr: Warn on incorrect string usage

The indentation stripping semantics of strings are fairly bad and have a
few gotchas where they behave unintuitively. But the good news is, that
these cases are easy to catch and can be avoided.
This commit adds a warning in the parser when such strings are detected.

Unfortunately Nixpkgs uses this kind of a lot, so we won't be able to
actually enable this warning for a while to come.

Co-authored-by: Commentator2.0 <lix@crystal-cavern.systems>
Change-Id: I3b3b68c2eee4cd70959d3f4ca643cb6caf3a2217

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

@@ -0,0 +1,13 @@
+[[test]]
+runner = "eval-okay"
+matrix = true
+flags = ["--extra-deprecated-features", "broken-string-indentation"]
+
+[[test]]
+name = "depr-warning"
+runner = "eval-okay"
+in = "in.nix"
+
+[[test]]
+runner = "parse-okay"
+flags = ["--extra-deprecated-features", "broken-string-indentation"]