Revert changes to pre-3.9 behavior, update version to 1.13.0

[DanielRosenwasser]

Fixes #105
Fixes #106

Here's the plan with this PR:

• Republish 1.11.2 as 1.13.0: this unbreaks people who haven't upgraded to TypeScript 3.9
• Publish 1.12.0 as 2.0.0: this means TypeScript 3.9 won't work on anything older than tslib 2.0

[imhoffd]

Will projects that want to use TypeScript 3.9 have to wait for transitive dependencies to also be updated to tslib 2.0?

[weswigham]

Will projects that want to use TypeScript 3.9 have to wait for transitive dependencies to also be updated to tslib 2.0?

Naw, with the minor we just published, the latest tslib 1.X should be compatible with TS 3.9, but not provide live bindings, so should also be compatible with emit from TS <3.9, while the 2.X version we're about to publish should be equivalent to the 1.12 we published, and will need TS 3.9+ output to function. So long as you and your dependencies declare at least the major version they depend on, everything should be fine.