tsconfig: Disable `strictNullChecks`, enable `strict`

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.

Start with tsconfig.json and compare its strictness settings with the linked MetaMask module template configuration. Update the configuration to match the requested strict-checking setup, then confirm the resulting TypeScript configuration is valid and the project still type-checks.

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