Have a test for the typescript peer dep to match upstream

[mightyiam]

https://github.com/standard/eslint-config-standard-with-typescript/blob/8c836dc44c4e8a1b73263259509df490c8e9fddf/package.json#L67

Does upstream support typescript@^3.9?

https://github.com/typescript-eslint/typescript-eslint#supported-typescript-version

I'd like us to have a test for this.

[mightyiam]

Actually, https://github.com/typescript-eslint/typescript-eslint#supported-typescript-version ... Upstream's attitude is to declare the support in the docs and then to declare * in the peerDep, "to allow experimentation". And their code prints a warning when the typescript version you're using is not supported. We can do the same, I suppose.