Enable `strictNullChecks` and `strictPropertyInitialization` in TS compilation

[justingrant]

strictNullChecks and strictPropertyInitialization are currently commented out in tsconfig.json. At some point we should fix the 300+ TS errors that show up with these flags enabled.

[jens-ox]

Hi Justin! If you want I can look into it. I wanted to start working with the new Temporal APIs, and this seems like a nice starting point. 😊

[justingrant]

Fixed in #109