codex: add custom search engines

App/Web Search/SearchProvider.swift

@@ -9,6 +9,18 @@ import Foundation
 
 class SearchProvider
 {
+    // Build a provider from a URL template. Template should contain %q or %s
+    // which will be replaced with the sanitized query string.
+    static func fromTemplate(_ template: String) -> SearchProvider {
+        SearchProvider(resolver: { query in
+            let sanitized = query.sanitized()
+            let replaced = template
+                .replacingOccurrences(of: "%q", with: sanitized)
+                .replacingOccurrences(of: "%s", with: sanitized)
+            return URL(string: replaced) ?? URL(string: "https://google.com/search?q=\(sanitized)&gbv=1")!
+        })
+    }
+
     static let google = SearchProvider(resolver: { query in
         // gbv=1: no JS
         URL(string: "https://google.com/search?q=\(query.sanitized())&gbv=1")!