Add a global title separator setting

### Description

Issue #161's MVP requirements call for the ability to customize the separator used in generated title tags. Today there is no dedicated separator setting anywhere in the codebase — the `$sep` parameter on `wp_title()` (`php/class-wp-seo.php`) is a vestigial doc-comment left over from WordPress core's `wp_title()` and is never actually used or output. Admins can only get a separator by manually typing a character into each per-page-type title format string (e.g. `#title# | #site_name#`), meaning changing the separator today requires hand-editing every `*_title` format field across every post type and archive section individually.

### Use Case

Site admins want to set the separator character (e.g. `|`, `-`, `•`) once, globally, and have it apply consistently across all generated title tags, the way comparable SEO plugins (Yoast, RankMath) handle it — rather than having to update dozens of individual title format fields by hand if they want to change it later.

### Proposed Solution

Add a `title_separator` setting (e.g. a dropdown of common separator characters, or a free-text field) plus a corresponding `#separator#` formatting tag, so it can be referenced from any title format string and changed globally in one place.

### Related

- #161 (parent MVP tracking issue)