Delete back/forward should remove mark when the character before/after has no such mark

### Given

- one marked character "a"
- cursor after/before the marked text
1. character before/after is not marked (supported by slate)
2. character before/after is not defined (block edges)


```tsx

```

### When

- delete back/forward
- press "a"

### Then

- "a" should be unmarked

```tsx

```
