Split notebookcell (#17185)

* Initial Split code

* minor change

* minor changes

* Added split cell button to initActionBar, created split cell class to pss cell context.

* added changes

* fixed index

* Split Cell Working in markdown mode

* Fixed highlighting

* Preserve the edit state

* Added new icon and updated styles and cellToolbar component with new icon name.

* Addressed PR

* Addressed PR

* Added back isEditMode flag

* Moved split action to after edit toggle.

* Fixed typo

* Addressed PR

* Addressed PR

* Removed deletion of the cell

* fixed the comments

Co-authored-by: Hale Rankin <harankin@microsoft.com>

src/sql/media/icons/common-icons.css

@@ -524,6 +524,15 @@ Includes non-masked style declarations. */
 	background-image: url('start-outline.svg');
 }
 
+.codicon:not(.masked-icon).icon-split-cell {
+	background-image: url("split_cell.svg");
+}
+
+.codicon.masked-icon.icon-split-cell:before {
+	-webkit-mask-image: url("split_cell.svg");
+	mask-image: url("split_cell.svg");
+}
+
 /* Masked element inside pseudo element */
 .masked-pseudo {
 	background-image: none !important;

src/sql/media/icons/split_cell.svg

@@ -0,0 +1,3 @@
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M1 1H14V14H1V1ZM13 13V8H2V13H13ZM13 7V2H2V7H13Z" fill="#323130"/>
+</svg>