Add support for markdown notebook renderers (#115191)

* Use shared webpack version instead of installing locally for simple-browser

* Use npm for building markdown preview

* render markdown in webview.

* update markdown preview height and offset

* Add basic custom notebook renderer point

* update css

* style update.

* update markdown header padding left

* Add example of loading katex to extend the markdown-it renderer

* Rename global to make clear it only applies to markdown-in

* hide/remove markdown preview

* Add wait for initial markdown preview rendering before showing notebook

* Add double click to switch to editing mode

* Fix markdown cells not getting updated after editing

* style polish

* notebook.experimental.useMarkdownRenderer

* switch render strategy.

* Adding very intial drag drop support for notebook markdown cells

* Implement drag/drop stubs for test classes

* Revert unrelated file changes

* Move markdown notebook math to own extension

* Add missing imports

Co-authored-by: rebornix <penn.lv@gmail.com>

extensions/markdown-notebook-math/.vscodeignore

@@ -0,0 +1,12 @@
+test/**
+test-workspace/**
+src/**
+tsconfig.json
+out/test/**
+out/**
+extension.webpack.config.js
+extension-browser.webpack.config.js
+cgmanifest.json
+yarn.lock
+preview-src/**
+webpack.config.js