tsconfig: Disable `strictNullChecks`, enable `strict`

[mcmire]

In the tsconfig.json in our module template, we have all of the strict options turned on instead of just null checks: https://github.com/MetaMask/metamask-module-template/blob/085b701d9084b45179caef9db1c0b60afa659a78/tsconfig.json#L12. We should do the same thing here.