(core) Adds cell context menu

Summary:
 - Brings in a new utility `contextMenu` to open context menu next to the mouse position
 - Use this utility to show a CellContextMenu, that sort of merge cell context menu and column context menu together.
 - Show cell context menu on context click on any grid's cell.
 - Also takes care of showing the row context menu for detail view on a context click that occurs on cells and not only on the row num header as it was the case prior to this diff.
 - task: https://gristlabs.getgrist.com/doc/check-ins/p/5#a1.s9.r1529.c31
 - discussion: https://grist.quip.com/ETGkAroLnc0Y/Cell-Context-Menu

{F40092}

Test Plan: - Adds project test and nbrowser for cell context menu and new cases for the detail row context menu.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3237

app/client/ui/RowContextMenu.ts

@@ -28,6 +28,7 @@ export function RowContextMenu({ disableInsert, disableDelete, isViewSorted }: I
   }
   result.push(
     menuDivider(),
+    // TODO: should show `Delete ${num} rows` when multiple are selected
     menuItemCmd(allCommands.deleteRecords, 'Delete',
       dom.cls('disabled', disableDelete)),
   );