LibJS: Start building a JavaScript engine for SerenityOS :^)

I always tell people to start building things by working on the thing
that seems the most interesting right now. The most interesting thing
here was an AST + simple interpreter, so that's where we start!

There is no lexer or parser yet, we build an AST directly and then
execute it in the interpreter, producing a return value.

This seems like the start of something interesting. :^)

Libraries/LibJS/Interpreter.cpp

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/AST.h>
+#include <LibJS/Interpreter.h>
+#include <LibJS/Object.h>
+#include <LibJS/Value.h>
+
+namespace JS {
+
+Interpreter::Interpreter()
+{
+    m_global_object = new Object;
+}
+
+Interpreter::~Interpreter()
+{
+}
+
+Value Interpreter::run(const ScopeNode& scope_node)
+{
+    enter_scope(scope_node);
+
+    Value last_value = js_undefined();
+    for (auto& node : scope_node.children()) {
+        last_value = node.execute(*this);
+    }
+
+    exit_scope(scope_node);
+    return last_value;
+}
+
+void Interpreter::enter_scope(const ScopeNode& scope_node)
+{
+    m_scope_stack.append({ scope_node });
+}
+
+void Interpreter::exit_scope(const ScopeNode& scope_node)
+{
+    ASSERT(&m_scope_stack.last().scope_node == &scope_node);
+    m_scope_stack.take_last();
+}
+
+void Interpreter::do_return()
+{
+    dbg() << "FIXME: Implement Interpreter::do_return()";
+}
+
+}