Add page-mode as a configuration option

### Is there an existing issue for this? - [x] I have searched the existing issues ### Problem Currently, `page_mode` (which handles `equal_width` and `equal_height` sizing) can only be triggered via shortcut mappings (e.g., `map S page_mode equal_width`). There is no corresponding `set` option in `zathurarc` to define a default behavior upon opening a document. For users who frequently work with documents containing inconsistent page sizes, it is cumbersome to manually trigger the page mode every time a file is opened. ### Expected behavior Add a `page-mode` (or `page_mode`) option to the settings. This would allow users to define the default sizing behavior in their `zathurarc`. ``` set page-mode equal_width ```