Generate exportable version of `workers-types`

[lrapoport-cf]

once overlap between workers and standard types is calculated (cloudflare/workerd#293):

• remove overlap from workers types
• produce separate package for overlapping types

[penalosa]

Following internal discussion, the scope of this issue has changed to producing a separate, exportable, entry point to the workers-types package, which developers can use in conjunction with lib.dom or lib.webworker