LibGUI: Some tweaks for TextEditor's will-execute-command virtual

Renamed the virtual from "on_edit_action" to "will_execute" so it
doesn't clash with our convention for Function hook names.

Also tighten the parameter type to GUI::TextDocumentUndoCommand
since that's the only kind of command it will receive.

Userland/DevTools/HackStudio/Editor.cpp

@@ -488,7 +488,7 @@ void Editor::LanguageServerAidedAutocompleteProvider::provide_completions(Functi
         data.value().position.column());
 }
 
-void Editor::on_edit_action(const GUI::Command& command)
+void Editor::will_execute(GUI::TextDocumentUndoCommand const& command)
 {
     if (!m_language_client)
         return;