typings: add missing sea binding properties

[hyemimi]

This updates the sea internal binding declarations to include properties that are already exposed by the native binding.

src/node_sea.cc registers getAssetKeys() and isVfsEnabled() on internalBinding('sea'):

SetMethod(context, target, "isVfsEnabled", IsVfsEnabled);
SetMethod(context, target, "getAssetKeys", GetAssetKeys);

It also conditionally exposes mainCodePath when running as a SEA with VFS enabled:

if (IsSingleExecutable()) {
  SeaResource sea_resource = FindSingleExecutableResource();
  if (static_cast<bool>(sea_resource.flags & SeaFlags::kEnableVfs)) {
    target
        ->Set(context,
              FIXED_ONE_BYTE_STRING(isolate, "mainCodePath"),
              code_path_str)
        .Check();
  }
}

These properties are used by existing internal JavaScript consumers, including lib/internal/vfs/sea.js, lib/internal/vfs/providers/sea.js, and lib/internal/main/embedding.js.

Adding the declarations keeps typings/internalBinding/sea.d.ts aligned with the native binding.