Store string position in the translation file

### Describe the problem

https://github.com/translate/translate/pull/5980 will add support for several formats to report the location of a string in the translation file. This would be useful for Weblate as well to better identify the string (not just by position) and to produce correct links for repository browsing.

### Solution brainstorm

I think it can be stored in the existing `position` field. The current index-based position will be used as a fallback for formats that do not support this.

### Describe alternatives you have considered

Add new field for storing the line number might be an option as well if existing one does not work.

### Screenshots

_No response_

### Additional context

_No response_