Playground: Enable automatic autocomplete in the editor

This makes it a bit more useful, as the user doesn't have to explicitly
ask for completion, it just provides completions, and tries really hard
to avoid suggesting things where they're not expected, for instance:
(cursor positions denoted as pipes)
```
@G | {|
    foo: bar |
    foo |
}
```

The user does not expect any suggestions in any of those cursor positions,
so provide no suggestions for such cases. This prevents the automatic autocomplete
getting in the way of the user, esp. when they try to press return fully
expecting to go to a new line.

DevTools/Playground/main.cpp

@@ -64,11 +64,13 @@ private:
         Vector<String> class_names;
         Vector<State> previous_states;
         bool should_push_state { true };
+        GUI::GMLToken* last_seen_token { nullptr };
 
         for (auto& token : all_tokens) {
             if (token.m_start.line > cursor.line() || (token.m_start.line == cursor.line() && token.m_start.column > cursor.column()))
                 break;
 
+            last_seen_token = &token;
             switch (state) {
             case Free:
                 if (token.m_type == GUI::GMLToken::Type::ClassName) {
@@ -102,9 +104,12 @@ private:
                 }
                 break;
             case InIdentifier:
-                state = AfterIdentifier;
+                if (token.m_type == GUI::GMLToken::Type::Colon)
+                    state = AfterIdentifier;
                 break;
             case AfterIdentifier:
+                if (token.m_type == GUI::GMLToken::Type::RightCurly || token.m_type == GUI::GMLToken::Type::LeftCurly)
+                    break;
                 if (token.m_type == GUI::GMLToken::Type::ClassMarker) {
                     previous_states.append(AfterClassName);
                     state = Free;
@@ -119,6 +124,11 @@ private:
         Vector<GUI::AutocompleteProvider::Entry> entries;
         switch (state) {
         case Free:
+            if (last_seen_token && last_seen_token->m_end.column + 1 != cursor.column() && last_seen_token->m_end.line == cursor.line()) {
+                // After some token, but with extra space, not on a new line.
+                // Nothing to put here.
+                break;
+            }
             GUI::WidgetClassRegistration::for_each([&](const GUI::WidgetClassRegistration& registration) {
                 entries.empend(String::formatted("@{}", registration.class_name()), 0u);
             });
@@ -126,6 +136,11 @@ private:
         case InClassName:
             if (class_names.is_empty())
                 break;
+            if (last_seen_token && last_seen_token->m_end.column + 1 != cursor.column() && last_seen_token->m_end.line == cursor.line()) {
+                // After a class name, but haven't seen braces.
+                // TODO: Suggest braces?
+                break;
+            }
             GUI::WidgetClassRegistration::for_each([&](const GUI::WidgetClassRegistration& registration) {
                 if (registration.class_name().starts_with(class_names.last()))
                     entries.empend(registration.class_name(), class_names.last().length());
@@ -134,6 +149,11 @@ private:
         case InIdentifier:
             if (class_names.is_empty())
                 break;
+            if (last_seen_token && last_seen_token->m_end.column + 1 != cursor.column() && last_seen_token->m_end.line == cursor.line()) {
+                // After an identifier, but with extra space
+                // TODO: Maybe suggest a colon?
+                break;
+            }
             if (auto registration = GUI::WidgetClassRegistration::find(class_names.last())) {
                 auto instance = registration->construct();
                 for (auto& it : instance->properties()) {
@@ -143,6 +163,13 @@ private:
             }
             break;
         case AfterClassName:
+            if (last_seen_token && last_seen_token->m_end.line == cursor.line()) {
+                if (last_seen_token->m_type != GUI::GMLToken::Type::Identifier || last_seen_token->m_end.column + 1 != cursor.column()) {
+                    // Inside braces, but on the same line as some other stuff (and not the continuation of one!)
+                    // The user expects nothing here.
+                    break;
+                }
+            }
             if (!class_names.is_empty()) {
                 if (auto registration = GUI::WidgetClassRegistration::find(class_names.last())) {
                     auto instance = registration->construct();
@@ -191,6 +218,7 @@ int main(int argc, char** argv)
 
     editor.set_syntax_highlighter(make<GUI::GMLSyntaxHighlighter>());
     editor.set_autocomplete_provider(make<GMLAutocompleteProvider>());
+    editor.set_should_autocomplete_automatically(true);
     editor.set_automatic_indentation_enabled(true);
     editor.set_text(R"~~~(@GUI::Widget {
     layout: @GUI::VerticalBoxLayout {