layout::par::collect: Treat linebreak element as whitespace (#1542)

Fixes #1540.

library/src/layout/par.rs

@@ -609,7 +609,10 @@ fn collect<'a>(
                         elem.text().chars().next()
                     } else if child.is::<SmartQuoteElem>() {
                         Some('"')
-                    } else if child.is::<SpaceElem>() || child.is::<HElem>() {
+                    } else if child.is::<SpaceElem>()
+                        || child.is::<HElem>()
+                        || child.is::<LinebreakElem>()
+                    {
                         Some(SPACING_REPLACE)
                     } else {
                         Some(OBJ_REPLACE)

tests/typ/bugs/smartquotes-on-newline.typ

@@ -0,0 +1,7 @@
+// Test that smart quotes are inferred correctly across newlines.
+
+---
+"test"#linebreak()"test"
+
+"test"\
+"test"