LibJS: Add alternative source-to-bytecode pipeline in Rust

Implement a complete Rust reimplementation of the LibJS frontend:
lexer, parser, AST, scope collector, and bytecode code generator.

The Rust pipeline is built via Corrosion (CMake-Cargo bridge) and
linked into LibJS as a static library. It is gated behind a build
flag (ENABLE_RUST, on by default except on Windows) and two runtime
environment variables:

- LIBJS_CPP: Use the C++ pipeline instead of Rust
- LIBJS_COMPARE_PIPELINES=1: Run both pipelines in lockstep,
  aborting on any difference in AST or bytecode generated.

The C++ side communicates with Rust through a C FFI layer
(RustIntegration.cpp/h) that passes source text to Rust and receives
a populated Executable back via a BytecodeFactory interface.

Libraries/LibJS/Rust/src/bytecode/instruction.rs

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+//! Bytecode instruction types generated from Bytecode.def.
+//!
+//! The `OpCode` enum, `Instruction` enum, and all encoding/visiting methods
+//! are generated by build.rs from Bytecode.def. This is the single source
+//! of truth for instruction definitions.
+
+include!(concat!(env!("OUT_DIR"), "/instruction_generated.rs"));