LibWeb+WebContent: Add abstraction layer for generic font families

Instead of hard-coding the names of system fonts to use for the CSS
generic fonts (like "sans-serif", "monospace", etc.) we now call out
to a Platform::FontPlugin and ask for the generic names.

Userland/Libraries/LibWeb/Platform/FontPlugin.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace Web::Platform {
+
+enum class GenericFont {
+    Cursive,
+    Fantasy,
+    Monospace,
+    SansSerif,
+    Serif,
+    UiMonospace,
+    UiRounded,
+    UiSansSerif,
+    UiSerif,
+    __Count,
+};
+
+class FontPlugin {
+public:
+    static FontPlugin& the();
+    static void install(FontPlugin&);
+
+    virtual ~FontPlugin();
+
+    virtual String generic_font_name(GenericFont) = 0;
+};
+
+}