layout: Do whitespace collapse during breaking and shaping (#31322)

This moves white space collapse to right before breaking and shaping
happens, which is more similar to what happens in legacy layout. This is
the first step toward making this procedure more efficient (avoiding
string copies) and also implementing support for `text-transform`.

Co-authored-by: Rakhi Sharma <atbrakhi@igalia.com>

components/layout_2020/tests/text.rs

@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+mod text {
+    use layout_2020::flow::text_run::WhitespaceCollapse;
+    use style::computed_values::white_space::T as WhiteSpace;
+
+    #[test]
+    fn test_collapse_whitespace() {
+        let collapse = |input, white_space, trim_beginning_white_space| {
+            WhitespaceCollapse::new(input, white_space, trim_beginning_white_space)
+                .collect::<String>()
+        };
+
+        let output = collapse("H ", WhiteSpace::Normal, false);
+        assert_eq!(output, "H ");
+
+        let output = collapse(" W", WhiteSpace::Normal, true);
+        assert_eq!(output, "W");
+
+        let output = collapse(" W", WhiteSpace::Normal, false);
+        assert_eq!(output, " W");
+
+        let output = collapse(" H  W", WhiteSpace::Normal, false);
+        assert_eq!(output, " H W");
+
+        let output = collapse("\n   H  \n \t  W", WhiteSpace::Normal, false);
+        assert_eq!(output, " H W");
+
+        let output = collapse("\n   H  \n \t  W   \n", WhiteSpace::Pre, false);
+        assert_eq!(output, "\n   H  \n \t  W   \n");
+
+        let output = collapse("\n   H  \n \t  W   \n ", WhiteSpace::PreLine, false);
+        assert_eq!(output, "\nH\nW\n");
+
+        let output = collapse("Hello \n World", WhiteSpace::PreLine, true);
+        assert_eq!(output, "Hello\nWorld");
+
+        let output = collapse(" \n World", WhiteSpace::PreLine, true);
+        assert_eq!(output, "\nWorld");
+
+        let output = collapse(" ", WhiteSpace::Normal, true);
+        assert_eq!(output, "");
+
+        let output = collapse(" ", WhiteSpace::Normal, false);
+        assert_eq!(output, " ");
+
+        let output = collapse("\n        ", WhiteSpace::Normal, true);
+        assert_eq!(output, "");
+
+        let output = collapse("\n        ", WhiteSpace::Normal, false);
+        assert_eq!(output, " ");
+    }
+}