LibWeb: Propagate input/textarea selection update to document selection

Calling `.setSelectionRange()` or `.select()` now updates the document
selection as well, visualizing the text selection.

Userland/Libraries/LibWeb/HTML/HTMLTextAreaElement.cpp

@@ -22,6 +22,7 @@
 #include <LibWeb/HTML/Numbers.h>
 #include <LibWeb/Infra/Strings.h>
 #include <LibWeb/Namespace.h>
+#include <LibWeb/Selection/Selection.h>
 
 namespace Web::HTML {
 
@@ -452,4 +453,13 @@ void HTMLTextAreaElement::queue_firing_input_event()
     });
 }
 
+void HTMLTextAreaElement::selection_was_changed()
+{
+    auto selection = document().get_selection();
+    if (!selection || selection->range_count() == 0)
+        return;
+
+    MUST(selection->set_base_and_extent(*m_text_node, selection_start().value(), *m_text_node, selection_end().value()));
+}
+
 }