LibWeb: Deduplicate the firing of input events in HTMLInputElement

Input elements without a defined user-interaction behavior need to fire
an input event when the user changes the element's value in some way.
This change moves the code to do this into its own function and adds
some spec text to explain what is being done.

Userland/Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -255,6 +255,8 @@ private:
     void handle_readonly_attribute(Optional<String> const& value);
     WebIDL::ExceptionOr<void> handle_src_attribute(String const& value);
 
+    void user_interaction_did_change_input_value();
+
     // https://html.spec.whatwg.org/multipage/input.html#value-sanitization-algorithm
     String value_sanitization_algorithm(String const&) const;