Investigate upgrading to tslib v2

[ecraig12345]

tslib@2 was recently released, and we may want to either upgrade to require that version or optionally add support for it in our dependency range. Need to investigate which TS versions are supported by tslib@2 both while compiling and for consumers and if it's possible to support both 1 and 2 for now.

[ecraig12345]

Per the tslib docs, the minimum typescript version to use tslib@2 is 3.9. So we can't and shouldn't use it until we internally upgrade to TS 3.9.

Per the tslib 2.0.0 release notes the only breaking change in version 2 is:

This release changes __exportStar and __importStar to use __createBinding which is incompatible with versions of TypeScript older than 3.9 (i.e. 3.8 and below).

Per this PR description:

TypeScript 3.9 won't work on anything older than tslib 2.0

From what I can tell (see this PR comment), the tslib version which must be used is bound to the TS version used to compile the library, not to consume it. So whenever we internally upgrade to TS 4.1, I think we should be able to update to tslib@2 at that time without any affect on consumers (besides a minor bundle size hit).

[ecraig12345]

Closing in favor of #16101 since it turns out this change won't be made until the overall TS 4 upgrade.