Add new strings as approved

### Describe the problem

The current [import methods](https://docs.weblate.org/en/latest/user/files.html#import-methods) support adding new strings, but there is no way to add new strings as approved.

### Describe the solution you would like

Add new option "Add new strings as approved".

### Describe alternatives you have considered

Maybe there could be a separate selection for method and target state, but that would not apply to some options (suggestions / replace / source).

### Screenshots

_No response_

### Additional context

_No response_