feat: add regex option for search bangs

[NecRaul]

This PR introduces a new optional regex field for search bangs, enabling advanced parsing of the user’s query. This allows bangs to extract multiple components from a query and substitute them into the final URL.

What this changes

• Adds regex: string to the bang configuration schema.
• Optional; preserves existing behavior if omitted.
• When provided, the regex is applied to the query after removing the bang shortcut.
• Updates:
  • Documentation (configuration.md)
  • HTML template (search.html)
  • Client-side behavior (page.js)
  • Bang struct (widget-search.go)

Why

This resolves issue #898, allowing certain shortcuts to extract multiple values from a query using a regular expression. This is especially useful for URLs that require structured query parameters, such as language codes or specific identifiers.