js: Allow for completion of lexically declared variables

This does require us to have a method which lists all the bindings in
a declarative environment which is not in the spec.

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp

@@ -142,4 +142,13 @@ void DeclarativeEnvironment::initialize_or_set_mutable_binding(Badge<ScopeNode>,
         set_mutable_binding(global_object, name, value, false);
 }
 
+Vector<String> DeclarativeEnvironment::bindings() const
+{
+    Vector<String> names;
+    for (auto& binding : m_bindings) {
+        names.empend(binding.key);
+    }
+    return names;
+}
+
 }

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.h

@@ -31,6 +31,9 @@ public:
 
     void initialize_or_set_mutable_binding(Badge<ScopeNode>, GlobalObject& global_object, FlyString const& name, Value value);
 
+    // This is not a method defined in the spec! Do not use this in any LibJS (or other spec related) code.
+    [[nodiscard]] Vector<String> bindings() const;
+
 protected:
     virtual void visit_edges(Visitor&) override;
 

Userland/Utilities/js.cpp

@@ -1361,6 +1361,12 @@ int main(int argc, char** argv)
             case CompleteVariable: {
                 auto const& variable = interpreter->global_object();
                 list_all_properties(variable.shape(), variable_name);
+
+                for (String& name : global_environment.declarative_record().bindings()) {
+                    if (name.starts_with(variable_name))
+                        results.empend(name);
+                }
+
                 if (results.size())
                     editor.suggest(variable_name.length());
                 break;