This changes some type definitions so that sheetjs accepts readonly arrays in places where it's not mutating the array anyway.
For example, without this change, the following code would not compile:
```
const myData: ReadonlyArray<{ col1: string, col2: string}> = [{ col1: 'a', col2: 'b' }];
const myWorksheet = utils. json_to_sheet(myData);
```
This theoretically should work - sheetjs isn't mutating the myData array anyway, so it should accept a readonly array as input.
Signed-off-by: Sweater-Baron <sweater-baron@noreply.git.sheetjs.com>
