(core) Support 'new' row in anchor links.

Summary:
- Anchor links with row of 'new' could be created but weren't parsed or used
  correctly. This fixes it.
- Also adds UIRowId type for row IDs which includes the special 'new' row. It's
  already been used in places as `number|'new'`, this diff gives it a name usable in app/common
  (it doesn't touch another name, RowId, that's been available in app/client).

Test Plan: Added a test assert for anchor links to new row

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3039

app/server/lib/Triggers.ts

@@ -65,7 +65,7 @@ export interface WebHookSecret {
 // An instance of this class should have .handle() called on it exactly once.
 export class TriggersHandler {
   // Converts a column ref to colId by looking it up in _grist_Tables_column
-  private _getColId: (rowId: (number | "new")) => string;
+  private _getColId: (rowId: number) => string|undefined;
 
   constructor(private _activeDoc: ActiveDoc) {
   }
@@ -82,7 +82,7 @@ export class TriggersHandler {
 
     const triggersByTableRef = _.groupBy(triggersTable.getRecords(), "tableRef");
     for (const [tableRef, triggers] of _.toPairs(triggersByTableRef)) {
-      const tableId = getTableId(Number(tableRef));  // groupBy makes tableRef a string
+      const tableId = getTableId(Number(tableRef))!;  // groupBy makes tableRef a string
       const tableDelta = summary.tableDeltas[tableId];
       if (!tableDelta) {
         continue;  // this table was not modified by these actions