Hackathon - Better Markdown Editor (#11540)

* Hackathon - better markdown editor - modified Bold to wrap selection in HTML. Split Image button into two new options: embed and link. Made preview container contentEditable.

* Removed the new dropdown from Image button -- it is not necessary since we are adding a context panel instead.

* Modified preview icons

* Set code-component dimensions so it is not visible. It is still being used to pass markdown changes to however.

* add turndown and save markdown

* update model on UI when source changes

* Added conditional that sets element attribute contentEditable when it is in edit mode.

* Added textView component that can be used for editing.

* update source on MD view not on every keystroke

* Added markdown editor buttons that allow user to swap editor, preview views.

* Cleaning up implementation

* Setting base value of _showPreview to false.

* don't allow html edit on split view

* Update editor automagically

* Add an image picking dialog to notebook toolbar.

* Await transformText()

* revert pushEditOperations to fix cursor issue

* Implemented radio buttons for three view toggles.

* Added new, optional properties to radioButton: name, icon class and tooltip. This allows for display as toggleable icon. Updated styles and theme accordingly.

* Style tweaks.

* Added new ViewAction file where the RadioButton action will reside.

* Removed radio button implementation in exchange for native button instantiation. Adjusted CSS and theme accordingly.

* Styles, component and template changes to handle view toggle between text, markdownn an splitview. Includes reverting of radioButton as this is no longer used.

* WYSIWYG 3 Modes

* Ensure one action active at a time

* Setting Text View button active by default. Cleaned up styles. Moved toolbar element to prevent code cell layout overflow.

* Ensure we respect editMode, add showMarkdown

* hiding overflow on code-cell

* Empty text container needs 100% width. Eliminates weird selection border too.

* Initialize _previewMode

* Actions Compatibility

* Further toolbar enhancements

* Update yarn lock after merge

* Slim down changes

* Remove commented out code

* Added margins around notebook-preview container for more visual space for text

* Add turndown to workbench html

* Tweak import

* Add types/turndown

* Remove workbench.html fix

* Import cjs modules directly for turndown

* Leverage solution from github

* browser umd

* non browser umd

* welp dependency

* Modified updatePreview to insert a p tag only when text cell is empty.

* add listener for undo

* add turndown to remote and web

* Fix workbench, check in plugin

* PR comment

Co-authored-by: maddydev <makoripa@microsoft.com>
Co-authored-by: chlafreniere <hichise@gmail.com>
Co-authored-by: Cory Rivera <corivera@microsoft.com>
Co-authored-by: Lucy Zhang <luczhan@microsoft.com>

remote/web/package.json

@@ -24,6 +24,8 @@
 		"sanitize-html": "^1.19.1",
 		"semver-umd": "^5.5.7",
 		"slickgrid": "github:anthonydresser/SlickGrid#2.3.33",
+    "turndown": "^6.0.0",
+    "turndown-plugin-gfm": "^1.0.2",
 		"vscode-oniguruma": "1.3.1",
 		"vscode-textmate": "5.2.0",
 		"xterm": "4.9.0-beta.8",