(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

test/nbrowser/gristUtils.ts

@@ -46,7 +46,7 @@ export const checkLoginPage = homeUtil.checkLoginPage.bind(homeUtil);
 export const fixturesRoot: string = testUtils.fixturesRoot;
 
 // it is sometimes useful in debugging to turn off automatic cleanup of docs and workspaces.
-const noCleanup = Boolean(process.env.NO_CLEANUP);
+export const noCleanup = Boolean(process.env.NO_CLEANUP);
 
 // Most test code uses simulateLogin through the server reference. Keep them to reduce unnecessary
 // code changes.
@@ -372,6 +372,10 @@ export async function dbClick(cell: WebElement) {
   await driver.withActions(a => a.doubleClick(cell));
 }
 
+export async function rightClick(cell: WebElement) {
+  await driver.withActions((actions) => actions.contextClick(cell));
+}
+
 /**
  * Returns {rowNum, col} object representing the position of the cursor in the active view
  * section. RowNum is a 1-based number as in the row headers, and col is a 0-based index for