Shell: Add runtime errors and implement break/continue

Such errors are raised when SyntaxError nodes are executed, and are also
used for internal control flow.
The 'break' and 'continue' commands are currently only allowed inside
for loops, and outside function bodies.

This also adds a 'loop' keyword for infinite loops.

Libraries/LibGUI/ShellSyntaxHighlighter.cpp

@@ -190,6 +190,13 @@ private:
         auto& span = span_for_node(node);
         span.color = m_palette.syntax_comment();
     }
+    virtual void visit(const AST::ContinuationControl* node) override
+    {
+        NodeVisitor::visit(node);
+
+        auto& span = span_for_node(node);
+        span.color = m_palette.syntax_control_keyword();
+    }
     virtual void visit(const AST::DynamicEvaluate* node) override
     {
         NodeVisitor::visit(node);