HackStudio: Add evaluate expression popup to debugger

This implements a dialog that can be used to evaluate a JS expression
in the HackStudio's Debugger context. It also implements simple
C++ Variable <-> JS Value conversion, allowing for JS expressions
to read/write variables in the debugger scope.

Currently, C++ structs are mapped to JS objects by way of a JS proxy,
however this leads to issues when printing, so this will be changed
in a later commit.

Userland/DevTools/HackStudio/Editor.h

@@ -103,6 +103,7 @@ private:
     bool m_hovering_editor { false };
     bool m_hovering_clickable { false };
     bool m_autocomplete_in_focus { false };
+    RefPtr<GUI::Action> m_evaluate_expression_action;
 
     OwnPtr<LanguageClient> m_language_client;
 };