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/Block.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/String.h>
+#include <LibJS/Bytecode/Block.h>
+#include <LibJS/Bytecode/Instruction.h>
+
+namespace JS::Bytecode {
+
+NonnullOwnPtr<Block> Block::create()
+{
+    return adopt_own(*new Block);
+}
+
+Block::Block()
+{
+}
+
+Block::~Block()
+{
+}
+
+void Block::append(Badge<Bytecode::Generator>, NonnullOwnPtr<Instruction> instruction)
+{
+    m_instructions.append(move(instruction));
+}
+
+void Block::dump() const
+{
+    for (size_t i = 0; i < m_instructions.size(); ++i) {
+        warnln("[{:3}] {}", i, m_instructions[i].to_string());
+    }
+}
+
+}