Turn off skipLibCheck in tsconfig

[dkundel]

The tsconfig.base.json file currently has skipLibCheck set to true (introduced in #450) which avoids throwing errors from TypeScript if one of the TypeScript definition files has an error. While this isn't a huge issue in terms of catching functional issues, it could introduce frustrations for customers who might use our own TypeScript definitions if those reference broken types.

In order to turn this off we need to make sure that after turning it off all relevant external types are updated and fixed if we have control over it.