Make type hints available to package consumers

[sigurdp]

The webviz-config package already has inline type hints but currently these are not available to package consumers. For example, the webviz-subsurface package, which has recently gained type hints will not be able to see the type hints from webviz-config. This means that running mypy linting on webviz-subsurface will not detect wrong usage of types imported from webviz-config since all imported types will be of type Any.

According to PEP 561 we need to add a py.typed marker file to our package and ensure that this file gets installed with the package.