Expose new `LoadingPageComponentType` and `ErrorPageComponentType` types

[joshpensky]

Description

This fixes an issue with the PageComponent type not allowing for the error prop, that's used for error page components

Related issues

Addresses #56

What this does

• Adds RouteError to export, removes Record<string, any> from RouteError type definition (this can only ever be a string or null)
• Adds new LoadingPageComponentType and ErrorPageComponentType types to describe the type of React component expected (added to export)
• Updates usage of PageComponent to use specific LoadingPageComponent or ErrorPageComponent types
• Updates intermediate example with NotFound to use error prop
• Removes unused stylelint reference

How to test

1. Pull down the repository and install (npm i, npm run bootstrap, npm start)
2. Visit the router config at /demos/intermediate/src/router/index.tsx
3. Confirm there is no error when passing the NotFound component into error prop
4. Visit the project http://localhost:3001
5. Visit a Missingno Pokémon. Confirm small error message appears under body text like so: