LibWeb: Implement CloseWatcher API

This implements most of the CloseWatcher API from the html spec.

AbortSignal support is unimplemented.

Integration with dialogs and popovers is also unimplemented.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -42,6 +42,7 @@ static bool is_platform_object(Type const& type)
         "CanvasGradient"sv,
         "CanvasPattern"sv,
         "CanvasRenderingContext2D"sv,
+        "CloseWatcher"sv,
         "CryptoKey"sv,
         "Document"sv,
         "DocumentType"sv,

Meta/gn/secondary/Userland/Libraries/LibWeb/HTML/BUILD.gn

@@ -22,6 +22,8 @@ source_set("HTML") {
     "CanvasPattern.cpp",
     "CanvasRenderingContext2D.cpp",
     "CloseEvent.cpp",
+    "CloseWatcher.cpp",
+    "CloseWatcherManager.cpp",
     "DOMParser.cpp",
     "DOMStringMap.cpp",
     "DataTransfer.cpp",

Meta/gn/secondary/Userland/Libraries/LibWeb/idl_files.gni

@@ -114,6 +114,7 @@ standard_idl_files = [
   "//Userland/Libraries/LibWeb/HTML/CanvasPattern.idl",
   "//Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.idl",
   "//Userland/Libraries/LibWeb/HTML/CloseEvent.idl",
+  "//Userland/Libraries/LibWeb/HTML/CloseWatcher.idl",
   "//Userland/Libraries/LibWeb/HTML/CustomElements/CustomElementRegistry.idl",
   "//Userland/Libraries/LibWeb/HTML/DataTransfer.idl",
   "//Userland/Libraries/LibWeb/HTML/DOMParser.idl",