Fix Emscripten 6 compatibility: restore ERRNO_CODES and bind FS method context

[holub]

Emscripten 6 made two breaking changes that break BrowserFS's EmscriptenFS shim:

1. ERRNO_CODES is no longer a global (removed in emscripten >=3.x). Pass FS, PATH, and an explicit errno map to EmscriptenFS so BrowserFS can map errno names to numbers.
2. FS node_ops/stream_ops methods are now called as bare functions (e.g. getattr(node) instead of node_ops.getattr(node)), losing . Bind all prototype methods on the node_ops and stream_ops wrapper instances so they retain their context.

Without these fixes, accessing files under /emulator throws:
TypeError: can't access property fs, this is undefined
at $.getattr → fstat → ___syscall_fstat64