Allow trailing comma in import statements (#8223)

Adding support for (optional) trailing commas like this:

    import {
        Foo,
        Bar,
    } from "foobar.slint";

This way it's more convenient to keep component imports sorted and
leads to smaller diffs when adding more components to the end of the
import statement.

ChangeLog: Allow trailing comma in import statements

Closes #4922

internal/compiler/parser/document.rs

@@ -313,8 +313,11 @@ fn parse_import_specifier(p: &mut impl Parser) -> bool {
 #[cfg_attr(test, parser_test)]
 /// ```test,ImportIdentifierList
 /// { Type1 }
-/// { Type2, Type3 }
-/// { Type as Alias1, Type as AnotherAlias }
+/// { Type2, }
+/// { Type3, Type4 }
+/// { Type5, Type6, }
+/// { Type as Alias1, Type as AnotherAlias1 }
+/// { Type as Alias2, Type as AnotherAlias2, }
 /// {}
 /// ```
 fn parse_import_identifier_list(p: &mut impl Parser) -> bool {
@@ -322,23 +325,14 @@ fn parse_import_identifier_list(p: &mut impl Parser) -> bool {
     if !p.expect(SyntaxKind::LBrace) {
         return false;
     }
-    if p.test(SyntaxKind::RBrace) {
-        return true;
-    }
     loop {
+        if p.test(SyntaxKind::RBrace) {
+            return true;
+        }
         parse_import_identifier(&mut *p);
-        match p.nth(0).kind() {
-            SyntaxKind::RBrace => {
-                p.consume();
-                return true;
-            }
-            SyntaxKind::Comma => {
-                p.consume();
-            }
-            _ => {
-                p.error("Expected comma or brace");
-                return false;
-            }
+        if !p.test(SyntaxKind::Comma) && p.nth(0).kind() != SyntaxKind::RBrace {
+            p.error("Expected comma or brace");
+            return false;
         }
     }
 }

internal/compiler/tests/syntax/imports/import_parse_error6.slint

@@ -0,0 +1,5 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+import { , SomeRect } from "../../typeloader/incpath/local_helper_type.slint";
+//       ^error{Syntax error: expected Identifier}

internal/compiler/tests/syntax/imports/import_parse_error7.slint

@@ -0,0 +1,5 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+import { SomeRect SomeRect as OtherRect } from "../../typeloader/incpath/local_helper_type.slint";
+//                ^error{Expected comma or brace}