- 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

TS 2.3.1 introduced a breaking change by [switching to covariant types for callbacks](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#covariance-in-callback-parameters). This change tries to fix these compiler errors in the workbench codebase

This reverts commit 698b6673.

fixes #23162

This change adds a new configuration setting that sets the default
language mode of new untitled files created with Ctrl+N
(workbench.action.files.newUntitledFile).  This activates the user's
desired language mode when a new untitled file is created.

Fixes #8729.

**Bug**
Can't view cat gifs in VSCode resource viewer. We only display images that are < 1mb :(

**Fix**
Add a more specific error message for large images and adds button to try opening the file in an external program