LibWeb: Allow inserting text at the cursor by typing characters :^)

This works everywhere right now, but it's obviously not going to stay
that way forever. :^)

Note that this does not advance the cursor correctly for whitespace
since the cursor is DOM-based and doesn't take whitespace collapsing
into account yet.

Libraries/LibWeb/DOM/Position.h

@@ -38,6 +38,8 @@ public:
 
     ~Position();
 
+    bool is_valid() const { return m_node; }
+
     Node* node() { return m_node; }
     const Node* node() const { return m_node; }