Change missing trailing comma span to point to span where comma is missing from

Signed-off-by: Torstein Grindvik <torstein.grindvik@nordicsemi.no>

packages/core-macro/tests/rsx.rs

@@ -0,0 +1,5 @@
+#[test]
+fn rsx() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/rsx/trailing-comma-0.rs");
+}

packages/core-macro/tests/rsx/trailing-comma-0.rs

@@ -0,0 +1,13 @@
+// Given an `rsx!` invocation with a missing trailing comma,
+// ensure the stderr output has an informative span.
+
+use dioxus::prelude::*;
+
+fn main() {
+    rsx! {
+        p {
+            class: "foo bar"
+            "Hello world"
+        }
+    };
+}

packages/core-macro/tests/rsx/trailing-comma-0.stderr

@@ -0,0 +1,5 @@
+error: missing trailing comma
+ --> tests/rsx/trailing-comma-0.rs:9:20
+  |
+9 |             class: "foo bar"
+  |                    ^^^^^^^^^