LibLocale+LibJS: Make a locale canonicalization API a bit more ergonomic

Instead of taking an out-parameter, return the canonicalization result.
This allows the API to be used where specs want to store the result and
the original values in separate variables.

Userland/Libraries/LibJS/Runtime/Intl/AbstractOperations.cpp

@@ -499,7 +499,7 @@ LocaleResult resolve_locale(Vector<String> const& requested_locales, LocaleOptio
         if (auto* options_string = options_value.has_value() ? options_value->get_pointer<String>() : nullptr) {
             // 1. Let optionsValue be the string optionsValue after performing the algorithm steps to transform Unicode extension values to canonical syntax per Unicode Technical Standard #35 LDML § 3.2.1 Canonical Unicode Locale Identifiers, treating key as ukey and optionsValue as uvalue productions.
             // 2. Let optionsValue be the string optionsValue after performing the algorithm steps to replace Unicode extension values with their canonical form per Unicode Technical Standard #35 LDML § 3.2.1 Canonical Unicode Locale Identifiers, treating key as ukey and optionsValue as uvalue productions.
-            ::Locale::canonicalize_unicode_extension_values(key, *options_string);
+            *options_string = ::Locale::canonicalize_unicode_extension_values(key, *options_string);
 
             // 3. If optionsValue is the empty String, then
             if (options_string->is_empty()) {

Userland/Libraries/LibLocale/Locale.cpp

@@ -491,7 +491,7 @@ String canonicalize_unicode_locale_id(StringView locale)
     return locale_data->to_string();
 }
 
-void canonicalize_unicode_extension_values(StringView key, String& value)
+String canonicalize_unicode_extension_values(StringView key, StringView value)
 {
     UErrorCode status = U_ZERO_ERROR;
 
@@ -507,7 +507,7 @@ void canonicalize_unicode_extension_values(StringView key, String& value)
     auto result = locale.getUnicodeKeywordValue<StringBuilder>(icu_string_piece(key), status);
     VERIFY(icu_success(status));
 
-    value = MUST(result.to_string());
+    return MUST(result.to_string());
 }
 
 StringView default_locale()

Userland/Libraries/LibLocale/Locale.h

@@ -128,7 +128,7 @@ Optional<LanguageID> parse_unicode_language_id(StringView);
 Optional<LocaleID> parse_unicode_locale_id(StringView);
 
 String canonicalize_unicode_locale_id(StringView);
-void canonicalize_unicode_extension_values(StringView key, String& value);
+String canonicalize_unicode_extension_values(StringView key, StringView value);
 
 StringView default_locale();
 bool is_locale_available(StringView locale);