LibJS: Add placeholder bytecode block sealing mechanism

After compiling bytecode, we should mark the memory read-only.
This currently does not work because it breaks instruction destruction.

I'm adding this anyway with a FIXME so we don't forget about it. :^)

Userland/Libraries/LibJS/Bytecode/Block.h

@@ -42,6 +42,7 @@ public:
     static NonnullOwnPtr<Block> create();
     ~Block();
 
+    void seal();
     void dump() const;
     ReadonlyBytes instruction_stream() const { return ReadonlyBytes { m_buffer, m_buffer_size }; }