initial support for Notebook extensibility. Fixes#3148 , Fixes#3382.
## Design notes
The extensibility patterns are modeled after the VSCode Document and Editor APIs but need to be different since core editor concepts are different - for example Notebooks have cells, and cells have contents rather than editors which have text lines.
Most importantly, a lot of the code is based on the MainThreadDocumentsAndEditors class, with some related classes (the MainThreadDocuments, and MainThreadEditors) brought in too. Given our current limitations I felt moving to add 3 full sets of extension host API classes was overkill so am currently using one. Will see if we need to change this in the future based on what we add in the additional APIs
## Limitations
The current implementation is limited to visible editors, rather than all documents in the workspace. We are not following the `openDocument` -> `showDocument` pattern, but instead just supporting `showDocument` directly.
## Changes in this PR
- Renamed existing APIs to make clear that they were about notebook contents, not about notebook behavior
- Added new APIs for querying notebook documents and editors
- Added new API for opening a notebook
- Moved `New Notebook` command to an extension, and added an `Open Notebook` command too
- Moved notebook feature flag to the extension
## Not covered in this PR
- Need to actually implement support for defining the provider and connection IDs for a notebook. this will be important to support New Notebook from a big data connection in Object Explorer
- Need to add APIs for adding cells, to support
- Need to implement the metadata for getting full notebook contents. I've only implemented to key APIs needed to make this all work.
