Allow ! in custom property values

See sass/dart-sass#260

spec/css/custom_properties/exclamation/expected_output.css

@@ -0,0 +1,8 @@
+.exclamation {
+  --important: value !important;
+  --multiple: !important !important;
+  --other-word: !something;
+  --in-identifier: foo!bar;
+  --just-exclam: !;
+  --just-exclams: !!!!!!!;
+}

spec/css/custom_properties/exclamation/input.scss

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+.exclamation {
+  // `!` is technically not allowed at the top-level of a custom property, but
+  // that's only because `!important` is filtered out before the custom property
+  // is parsed by the CSS parser. As far as Sass is concerned, it's fine.
+  --important: value !important;
+
+  // We even allow constructions like these for forwards-compatibility with
+  // additional flags or syntax CSS might add.
+  --multiple: !important !important;
+  --other-word: !something;
+  --in-identifier: foo!bar;
+  --just-exclam: !;
+  --just-exclams: !!!!!!!;
+}

spec/css/custom_properties/exclamation/options.yml

@@ -0,0 +1,3 @@
+---
+:todo:
+- libsass