(core) Adding sort options for columns.

Summary:
Adding sort options for columns.
- Sort menu has a new option "More sort options" that opens up Sort left menu
- Each sort entry has an additional menu with 3 options
-- Order by choice index (for the Choice column, orders by choice position)
-- Empty last (puts empty values last in ascending order, first in descending order)
-- Natural sort (for Text column, compares strings with numbers as numbers)
Updated also CSV/Excel export and api sorting.
Most of the changes in this diff is a sort expression refactoring. Pulling out all the methods
that works on sortExpression array into a single namespace.

Test Plan: Browser tests

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: dsagal, alexmojaki

Differential Revision: https://phab.getgrist.com/D3077

app/server/lib/ServerColumnGetters.ts

@@ -1,5 +1,8 @@
-import {ColumnGetters} from 'app/common/ColumnGetters';
+import { ColumnGetter, ColumnGetters } from 'app/common/ColumnGetters';
 import * as gristTypes from 'app/common/gristTypes';
+import { safeJsonParse } from 'app/common/gutil';
+import { choiceGetter } from 'app/common/SortFunc';
+import { Sort } from 'app/common/SortSpec';
 
 /**
  *
@@ -12,23 +15,33 @@ export class ServerColumnGetters implements ColumnGetters {
   private _colIndices: Map<number, string>;
 
   constructor(rowIds: number[], private _dataByColId: {[colId: string]: any}, private _columns: any[]) {
-    this._rowIndices = new Map<number, number>(rowIds.map((rowId, r) => [rowId, r] as [number, number]));
+    this._rowIndices = new Map<number, number>(rowIds.map((rowId, index) => [rowId, index] as [number, number]));
     this._colIndices = new Map<number, string>(_columns.map(col => [col.id, col.colId] as [number, string]));
   }
 
-  public getColGetter(colRef: number): ((rowId: number) => any) | null {
+  public getColGetter(colSpec: Sort.ColSpec): ColumnGetter | null {
+    const colRef = Sort.getColRef(colSpec);
     const colId = this._colIndices.get(colRef);
     if (colId === undefined) {
       return null;
     }
     const col = this._dataByColId[colId];
-    return rowId => {
+    let getter = (rowId: number) => {
       const idx = this._rowIndices.get(rowId);
       if (idx === undefined) {
         return null;
       }
       return col[idx];
     };
+    const details = Sort.specToDetails(colSpec);
+    if (details.orderByChoice) {
+      const rowModel = this._columns.find(c => c.id == colRef);
+      if (rowModel?.type === 'Choice') {
+        const choices: string[] = safeJsonParse(rowModel.widgetOptions, {}).choices || [];
+        getter = choiceGetter(getter, choices);
+      }
+    }
+    return getter;
   }
 
   public getManualSortGetter(): ((rowId: number) => any) | null {