LibCore: Implement four-digit modes for FilePermissionsMask parsing

Author: https://github.com/timschumi
Commit: 6c4b5775e1
Pull-request: https://github.com/SerenityOS/serenity/pull/14685
Reviewed-by: https://github.com/bgianfo ✅

Tests/LibCore/TestLibCoreFilePermissionsMask.cpp

@@ -116,3 +116,24 @@ TEST_CASE(file_permission_mask_parse)
     mask = Core::FilePermissionsMask::parse("z+rw"sv);
     EXPECT(mask.is_error());
 }
+
+TEST_CASE(numeric_mask_special_bits)
+{
+    {
+        auto mask = Core::FilePermissionsMask::parse("750"sv);
+        EXPECT(!mask.is_error());
+        EXPECT_EQ(mask.value().apply(07000), 07750);
+    }
+
+    {
+        auto mask = Core::FilePermissionsMask::parse("7750"sv);
+        EXPECT(!mask.is_error());
+        EXPECT_EQ(mask.value().apply(0), 07750);
+    }
+
+    {
+        auto mask = Core::FilePermissionsMask::parse("0750"sv);
+        EXPECT(!mask.is_error());
+        EXPECT_EQ(mask.value().apply(07000), 0750);
+    }
+}