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/Generator.cpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/OwnPtr.h>
+#include <LibJS/AST.h>
+#include <LibJS/Bytecode/Block.h>
+#include <LibJS/Bytecode/Generator.h>
+#include <LibJS/Bytecode/Instruction.h>
+#include <LibJS/Bytecode/Register.h>
+#include <LibJS/Forward.h>
+
+namespace JS::Bytecode {
+
+Generator::Generator()
+{
+    m_block = Block::create();
+}
+
+Generator::~Generator()
+{
+}
+
+OwnPtr<Block> Generator::generate(ASTNode const& node)
+{
+    Generator generator;
+    [[maybe_unused]] auto dummy = node.generate_bytecode(generator);
+    generator.m_block->set_register_count({}, generator.m_next_register);
+    return move(generator.m_block);
+}
+
+void Generator::append(NonnullOwnPtr<Instruction> instruction)
+{
+    m_block->append({}, move(instruction));
+}
+
+Register Generator::allocate_register()
+{
+    VERIFY(m_next_register != NumericLimits<u32>::max());
+    return Register { m_next_register++ };
+}
+
+}