Add opt-in debugger protocol v2 support for Godot 3.6 and 4.6

[nowasm]

Summary

Add opt-in dual-channel debugger protocol v2 adapters for Godot 3.6 and 4.6. The extension selects the Godot 3 or Godot 4 adapter from the project version. The new debug_protocol_v2 launch/attach setting selects v2 only when explicitly enabled; unmodified Godot builds continue to use the legacy debugger by default.

The v2 client supports separate control/data channels, acknowledged execution commands, stop-scoped lazy stack and variable requests, variable paging, cancellation, reconnection generation checks, and matching engine launch endpoints.

Related engine PRs

• Godot 3.6: Debugger: Add dual-channel protocol v2 for Godot 3.6 godot#124372
• Godot 4.6: Debugger: Add opt-in dual-channel protocol v2 for Godot 4.6 godot#124373

A matching custom engine build is required to enable v2. This PR can remain a draft while the engine PRs are reviewed.

Verification

• TypeScript compile and lint pass (lint reports only pre-existing warnings).
• 21 unit tests pass for protocol framing, session state, and adapter selection.
• Godot 3.6 v2 integration: 2 tests pass, including 100 acknowledged steps, lazy variable paging, and same-line call stepping.
• Godot 4.6 v2 integration: 1 test passes for channel pairing, variable paging, and 20 acknowledged steps.
• Stock Godot 4.6 debugger regression: 8 tests pass.
• Custom-engine integration tests are excluded from the default test glob.

The stock Godot 3 adapter source remains unchanged, and selection tests confirm it remains the default.