LibJS: Introduce an accumulator register to Bytecode::Interpreter

This commit introduces the concept of an accumulator register to
LibJS's bytecode interpreter. The accumulator register is always
register 0, and most simple instructions use it for reading and
writing.

Not only does this slim down the AST, but it also simplifies a lot of
the code. For example, the generate_bytecode methods no longer need
to return an Optional<Register>, as any opcode which has a "return"
value will always put it into the accumulator.

This also renames the old Op::Load to Op::LoadImmediate, and uses
Op::Load to load from a register into the accumulator. There is
also an Op::Store to put the value in the accumulator into another
register.

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -26,7 +26,7 @@ Generator::~Generator()
 OwnPtr<Block> Generator::generate(ASTNode const& node)
 {
     Generator generator;
-    [[maybe_unused]] auto dummy = node.generate_bytecode(generator);
+    node.generate_bytecode(generator);
     generator.m_block->set_register_count({}, generator.m_next_register);
     generator.m_block->seal();
     return move(generator.m_block);