Don't export Future as a const enum

[yinonburgansky]

Future is exported a const enum, causing an error Cannot access ambient const enums when 'isolatedModules' is enabled.
see https://ncjamieson.com/dont-export-const-enums/ for more info.

this is needed for solid-start:
solidjs/solid-start#1968

[lxsmnsyc]

hmmm, I thought I configured this to still compile into how normal enums work. I guess I can revert that. Will check thanks