Update TypeScript config to enable useDefineForClassFields

[GytisCepk]

useDefineForClassFields changes behavior of how class properties are initialized (TypeScript documentation). TypeScript recommends enabling it as it makes TypeScript code closer to ECMA runtime behavior

This breaks following:

• All classes derived from Entity:
  Because of how properties are initialized in Entity class, all classes which extend Entity and initialize it's properties via Entity constructor (super(props, iModel)) will not have those properties initialized (they will be set to undefined). TS playground of suggested fix
• A few tests in @itwin/core-bentley:
  With useDefineForClassFields enabled properties not initialized during class construction are created and set to undefined while currently those properties are absent. This breaks some of the deepEqual asserts.
• RPC interface / implementation registration.