LibWeb: Move Timer from DOM directory & namespace to HTML

Timers are part of the HTML spec. :^)
https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timers

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -98,7 +98,6 @@ set(SOURCES
     DOM/StaticRange.cpp
     DOM/Text.cpp
     DOM/Text.idl
-    DOM/Timer.cpp
     DOMParsing/InnerHTML.cpp
     DOMTreeModel.cpp
     Dump.cpp
@@ -213,6 +212,7 @@ set(SOURCES
     HTML/SyntaxHighlighter/SyntaxHighlighter.cpp
     HTML/TagNames.cpp
     HTML/TextMetrics.cpp
+    HTML/Timer.cpp
     HTML/Window.cpp
     HTML/Worker.cpp
     HTML/WorkerDebugConsoleClient.cpp