pure internal modules + external shim

- The top-level modules Express and NodeJs are now 'pure' ie
non-instantiated, due to removal of all vars, functions, and classes
- Where the above change breaks existing typings which access types via
external module references, a shim has been added to make this continue
working unchanged
- A single namespace _ExternalShim_ is added but this can be reused
across all typings that need shimming