LibWeb: Factor out CanvasSettings mixin into separate file

This refactors this mixin, that was introduced in #4506 to be the same
as all other mixins, so that it can be used for #3788

Libraries/LibWeb/HTML/Canvas/CanvasSettings.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2025, Ladybird contributors
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Web::HTML {
+
+struct CanvasRenderingContext2DSettings {
+    bool alpha { true };
+    bool desynchronized { false };
+    Bindings::PredefinedColorSpace color_space { Bindings::PredefinedColorSpace::Srgb };
+    Bindings::CanvasColorType color_type { Bindings::CanvasColorType::Unorm8 };
+    bool will_read_frequently { false };
+};
+
+// https://html.spec.whatwg.org/multipage/canvas.html#canvassettings
+class CanvasSettings {
+public:
+    virtual ~CanvasSettings() = default;
+
+    virtual CanvasRenderingContext2DSettings get_context_attributes() const = 0;
+
+protected:
+    CanvasSettings() = default;
+};
+
+}

Libraries/LibWeb/HTML/Canvas/CanvasSettings.idl

@@ -0,0 +1,5 @@
+// https://html.spec.whatwg.org/multipage/canvas.html#canvassettings
+interface mixin CanvasSettings {
+    // settings
+    CanvasRenderingContext2DSettings getContextAttributes();
+};