Add types for error constructors

[mthadley]

I was recently writing some tests for handling errors from openid-client and while attempting to instantiate one of the errors, I noticed that the constructor types don't match the implementation.

Since the types don't explicitly declare a constructor, they inherit they inherit it from Error, which doesn't match their overloaded constructors.

So, I tried adding what I think are roughly the correct constructor types for the OPError and RPError classes. I saw that there is a openid-client-tests.ts file, so let me know if there's anything additional I should add there as well. Or, feel free to push any additional updates that you see fit to this branch.

Thanks!

[panva]

Thank you @mthadley