LibJS: Start fleshing out a bytecode for the JavaScript engine :^)

This patch begins the work of implementing JavaScript execution in a
bytecode VM instead of an AST tree-walk interpreter.

It's probably quite naive, but we have to start somewhere.

The basic idea is that you call Bytecode::Generator::generate() on an
AST node and it hands you back a Bytecode::Block filled with
instructions that can then be interpreted by a Bytecode::Interpreter.

This first version only implements two instructions: Load and Add. :^)

Each bytecode block has infinity registers, and the interpreter resizes
its register file to fit the block being executed.

Two new `js` options are added in this patch as well:

`-d` will dump the generated bytecode
`-b` will execute the generated bytecode

Note that unless `-d` and/or `-b` are specified, none of the bytecode
related stuff in LibJS runs at all. This is implemented in parallel
with the existing AST interpreter. :^)

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Bytecode/Interpreter.h>
+#include <LibJS/Bytecode/Op.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS::Bytecode::Op {
+
+void Load::execute(Bytecode::Interpreter& interpreter) const
+{
+    interpreter.reg(m_dst) = m_value;
+}
+
+void Add::execute(Bytecode::Interpreter& interpreter) const
+{
+    interpreter.reg(m_dst) = add(interpreter.global_object(), interpreter.reg(m_src1), interpreter.reg(m_src2));
+}
+
+String Load::to_string() const
+{
+    return String::formatted("Load dst:r{}, value:{}", m_dst.index(), m_value.to_string_without_side_effects());
+}
+
+String Add::to_string() const
+{
+    return String::formatted("Add dst:r{}, src1:r{}, src2:r{}", m_dst.index(), m_src1.index(), m_src2.index());
+}
+
+}