[PERF] history: faster history at import

The history is sometimes used in plugin `import` methods:
`this.history.update(...)`

There are 2 main reason to do that:
- data structures are typed as readonly (to force using the history
  when handling commands)
- it's easier since it creates all intermediary objects itself
  `this.history.update("myData", sheetId, pivotId, value)`, if `myData`
  is empty, it will creates intermediary objects with keys `sheetId`
  and `pivotId`: `myData: { [sheetId]: { [pivotId]: data } }`

At import however, keeping track of the changes is useless.
They are currently aggregated in the `changes` array, but never used.

This commit improves loading time of the large number data set by ~17%

Before: 1353ms
After:	1116ms

(averages over 8 runs)

closes #4732

Task: 4083517
X-original-commit: 3868552
Signed-off-by: Rémi Rahir (rar) <[email protected]>
Signed-off-by: Lucas Lefèvre (lul) <[email protected]>

Author: LucasLefevre

src/state_observer.ts

@@ -4,7 +4,7 @@ import { CoreCommand, HistoryChange } from "./types";
 type HistoryPath = [any, ...(number | string)[]];
 
 export class StateObserver {
-  private changes: HistoryChange[] = [];
+  private changes: HistoryChange[] | undefined;
   private commands: CoreCommand[] = [];
 
   /**
@@ -39,7 +39,7 @@ export class StateObserver {
     if (value[key] === val) {
       return;
     }
-    this.changes.push({
+    this.changes?.push({
       key,
       target: value,
       before: value[key],