Add a mechanism to be a customer of the extension host (share lifecycle) and begin reducing usage of IThreadService

- Initial version of custom views and tree data provider API

- Adopt proposed API to contribute explorer views
- Adopt extension host and main to contribute explorer views
- Implement contributable views in explorer viewlet

- API to create a view passing the data provider
- Have methods on created API to refresh

- A new approach to register tree node
- Support on change listener to refresh node

Load configuration for languages when they are first encountered as an embedded mode in TextMate (#14136)

debt - no guessing-caching, better use of gc-signals, command converter using gc-signals, main side heap service