PC-Axis (.px) file support

### Describe the problem

.px files are commonly used to exchange statistical data
https://www.scb.se/en/services/statistical-programs-for-px-files/
and they can and do contain translations for values (eg "female", "older than 65", "fossil fuel", "coal", "Germany", "poultry"...):
https://www.scb.se/en/services/statistical-programs-for-px-files/px-file-format/

### Describe the solution you'd like

Allow .px files to be translated via WebLate

### Describe alternatives you've considered

Keeping translations separate (keys possibly getting out-of sync, non-standard)
Editing with other tools (no benefit of translation memory, translators have to learn those tools...)

### Screenshots

_No response_

### Additional context

_No response_