(core) Allow filtering by selected cell value in cell context menu

Summary: Adds a command and `BaseView` method `filterByThisCellValue`.

Test Plan: Added two tests to `nbrowser/CellContextMenu.ts`

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3383

app/client/ui/CellContextMenu.ts

@@ -43,7 +43,9 @@ export function CellContextMenu(rowOptions: IRowContextMenu, colOptions: IMultiC
     ...(
       (numCols > 1 || numRows > 1) ? [] : [
         menuDivider(),
-        menuItemCmd(allCommands.copyLink, 'Copy anchor link')
+        menuItemCmd(allCommands.copyLink, 'Copy anchor link'),
+        menuDivider(),
+        menuItemCmd(allCommands.filterByThisCellValue, `Filter by this value`),
       ]
     ),
 

app/client/ui/ColumnFilterMenu.ts

@@ -407,6 +407,7 @@ interface ICountOptions {
  *
  * The optional column type controls how complex cell values are decomposed into keys (e.g. Choice Lists have
  * the possible choices as keys).
+ * Note that this logic is replicated in BaseView.prototype.filterByThisCellValue.
  */
 function addCountsToMap(valueMap: Map<CellValue, IFilterCount>, rowIds: RowId[],
                         { keyMapFunc = identity, labelMapFunc = identity, columnType,