[wgsl] Add parser error snapshot

tests/errors.rs

@@ -0,0 +1,15 @@
+#[cfg(feature = "wgsl-in")]
+macro_rules! err {
+    ($value:expr, @$snapshot:literal) => {
+        ::insta::assert_snapshot!(naga::front::wgsl::parse_str($value).expect_err("expected parser error").to_string(), @$snapshot);
+    };
+}
+
+#[cfg(feature = "wgsl-in")]
+#[test]
+fn function_without_identifier() {
+    err!(
+        "fn () {}",
+        @"error while parsing WGSL in scopes [FunctionDecl] at line 1 pos 4: unexpected token Paren('('), expected ident"
+    );
+}