Add cssnano

The CSS minifier we currently use (clean-css) is not maintained and
lacks support for new CSS features. CSSNano is a replacement that
supports features we need (primarily container queries).

This CL only adds the relevant dependencies, and does not use it. I will
do that (and remove clean-css) in a follow-up.

Bug: 1399763
Change-Id: I899ecc4482164404dd5ac2fd13f92fb797978e29
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/4092305
Commit-Queue: Jack Franklin <jacktfranklin@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Jack Franklin <jacktfranklin@chromium.org>
diff --git a/node_modules/postcss-selector-parser/postcss-selector-parser.d.ts b/node_modules/postcss-selector-parser/postcss-selector-parser.d.ts
index 383e222..89a2c52 100644
--- a/node_modules/postcss-selector-parser/postcss-selector-parser.d.ts
+++ b/node_modules/postcss-selector-parser/postcss-selector-parser.d.ts
@@ -339,8 +339,16 @@
     function selector(opts: ContainerOptions): Selector;
     function isSelector(node: any): node is Selector;
 
+    interface CombinatorRaws {
+        value?: string;
+        spaces?: {
+            before?: string;
+            after?: string;
+        };
+    }
     interface Combinator extends Base {
-        type: "combinator"
+        type: "combinator";
+        raws?: CombinatorRaws;
     }
     function combinator(opts: NodeOptions): Combinator;
     function isCombinator(node: any): node is Combinator;