LibWeb: Separate text control input events handling from contenteditable

This input event handling change is intended to address the following
design issues:
- Having `DOM::Position` is unnecessary complexity when `Selection`
  exists because caret position could be described by the selection
  object with a collapsed state. Before this change, we had to
  synchronize those whenever one of them was modified, and there were
  already bugs caused by that, i.e., caret position was not changed when
  selection offset was modified from the JS side.
- Selection API exposes selection offset within `<textarea>` and
  `<input>`, which is not supposed to happen. These objects should
  manage their selection state by themselves and have selection offset
  even when they are not displayed.
- `EventHandler` looks only at `DOM::Text` owned by `DOM::Position`
  while doing text manipulations. It works fine for `<input>` and
  `<textarea>`, but `contenteditable` needs to consider all text
  descendant text nodes; i.e., if the cursor is moved outside of
  `DOM::Text`, we need to look for an adjacent text node to move the
  cursor there.

With this change, `EventHandler` no longer does direct manipulations on
caret position or text content, but instead delegates them to the active
`InputEventsTarget`, which could be either
`FormAssociatedTextControlElement` (for `<input>` and `<textarea>`) or
`EditingHostManager` (for `contenteditable`). The `Selection` object is
used to manage both selection and caret position for `contenteditable`,
and text control elements manage their own selection state that is not
exposed by Selection API.

This change improves text editing on Discord, as now we don't have to
refocus the `contenteditable` element after character input. The problem
was that selection manipulations from the JS side were not propagated
to `DOM::Position`.

I expect this change to make future correctness improvements for
`contenteditable` (and `designMode`) easier, as now it's decoupled from
`<input>` and `<textarea>` and separated from `EventHandler`, which is
quite a busy file.

Userland/Libraries/LibWeb/HTML/FormAssociatedElement.h

@@ -10,6 +10,7 @@
 #include <AK/String.h>
 #include <AK/WeakPtr.h>
 #include <LibWeb/Bindings/HTMLFormElementPrototype.h>
+#include <LibWeb/DOM/InputEventsTarget.h>
 #include <LibWeb/Forward.h>
 #include <LibWeb/WebIDL/Types.h>
 
@@ -96,7 +97,7 @@ public:
     HTMLElement const& form_associated_element_to_html_element() const { return const_cast<FormAssociatedElement&>(*this).form_associated_element_to_html_element(); }
 
     // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-form-reset-control
-    virtual void reset_algorithm() {};
+    virtual void reset_algorithm() { }
 
     virtual void clear_algorithm();
 
@@ -129,7 +130,8 @@ enum class SelectionSource {
     DOM,
 };
 
-class FormAssociatedTextControlElement : public FormAssociatedElement {
+class FormAssociatedTextControlElement : public FormAssociatedElement
+    , public InputEventsTarget {
 public:
     // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-textarea/input-relevant-value
     virtual String relevant_value() = 0;
@@ -168,13 +170,34 @@ public:
     bool has_scheduled_selectionchange_event() const { return m_has_scheduled_selectionchange_event; }
     void set_scheduled_selectionchange_event(bool value) { m_has_scheduled_selectionchange_event = value; }
 
+    virtual JS::GCPtr<DOM::Text> form_associated_element_to_text_node() = 0;
+    virtual JS::GCPtr<DOM::Text const> form_associated_element_to_text_node() const { return const_cast<FormAssociatedTextControlElement&>(*this).form_associated_element_to_text_node(); }
+
+    virtual void handle_insert(String const&) override;
+    virtual void handle_delete(DeleteDirection) override;
+    virtual void handle_return_key() override;
+    virtual void select_all() override;
+    virtual void set_selection_anchor(JS::NonnullGCPtr<DOM::Node>, size_t offset) override;
+    virtual void set_selection_focus(JS::NonnullGCPtr<DOM::Node>, size_t offset) override;
+    virtual void move_cursor_to_start(CollapseSelection) override;
+    virtual void move_cursor_to_end(CollapseSelection) override;
+    virtual void increment_cursor_position_offset(CollapseSelection) override;
+    virtual void decrement_cursor_position_offset(CollapseSelection) override;
+    virtual void increment_cursor_position_to_next_word(CollapseSelection) override;
+    virtual void decrement_cursor_position_to_previous_word(CollapseSelection) override;
+
+    JS::GCPtr<DOM::Position> cursor_position() const;
+
 protected:
     // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-textarea/input-relevant-value
-    void relevant_value_was_changed(JS::GCPtr<DOM::Text>);
+    void relevant_value_was_changed();
 
     virtual void selection_was_changed([[maybe_unused]] size_t selection_start, [[maybe_unused]] size_t selection_end) { }
 
 private:
+    void collapse_selection_to_offset(size_t);
+    void selection_was_changed();
+
     // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#concept-textarea/input-selection
     WebIDL::UnsignedLong m_selection_start { 0 };
     WebIDL::UnsignedLong m_selection_end { 0 };