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/Interpreter.cpp

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Bytecode/Block.h>
+#include <LibJS/Bytecode/Instruction.h>
+#include <LibJS/Bytecode/Interpreter.h>
+
+namespace JS::Bytecode {
+
+Interpreter::Interpreter(GlobalObject& global_object)
+    : m_global_object(global_object)
+{
+}
+
+Interpreter::~Interpreter()
+{
+}
+
+void Interpreter::run(Bytecode::Block const& block)
+{
+    dbgln("Bytecode::Interpreter will run block {:p}", &block);
+
+    m_registers.resize(block.register_count());
+
+    for (auto& instruction : block.instructions())
+        instruction.execute(*this);
+
+    dbgln("Bytecode::Interpreter did run block {:p}", &block);
+    for (size_t i = 0; i < m_registers.size(); ++i) {
+        String value_string;
+        if (m_registers[i].is_empty())
+            value_string = "(empty)";
+        else
+            value_string = m_registers[i].to_string_without_side_effects();
+        dbgln("[{:3}] {}", i, value_string);
+    }
+}
+
+}