Brace completion changes outer list to array in error

Brace completion changes outer list to array in error.

Take an example like:

```fs
let x = [
[| (* start a new array before this element *) box 1; box"some key"|]
] // outer list is changed to array
```

Resulting in broken code after brace completion.
```fs
let x = [
[| [|box 1; box"some key"|]
|]
```

Another way to trigger it is:
```fs
let x = [(*start a new array directly before this array open, leaving no whitespace*)[||]]
```

Start with the brace-completion behavior for nested F# list and array delimiters, using the two examples in the issue to reproduce the problem. Done means completing the inner array no longer changes the outer list into an array, including when there is no whitespace between the openings.

Written by the indexing model from the issue text.