Fix BaseTooltip.edit() signature (slab#3816)

At the moment, the `BaseTooltip.edit()` signature has type:

```ts
declare class BaseTooltip extends Tooltip {
  edit(mode?: string, preview?: null): void;
}
```

This change tweaks the signature to be more accurate:

```ts
declare class BaseTooltip extends Tooltip {
  edit(mode?: string, preview?: string | null): void;
}
```

themes/base.ts

@@ -237,7 +237,7 @@ class BaseTooltip extends Tooltip {
  this.hide();
  }
 
- edit(mode = 'link', preview = null) {
+ edit(mode = 'link', preview: string | null = null) {
  this.root.classList.remove('ql-hidden');
  this.root.classList.add('ql-editing');
  if (preview != null) {